“”Just in case they don't see the light, I suggest we take their guns away before they come in.
|—"Jim Bowie" in Comanche Territory|
A gun is a device for launching projectiles at targets one wishes to damage, injure, or kill. They come in all lengths and calibers, with projectiles made of many materials, for various purposes. The principal uses for the gun are war, hunting, and target shooting, self-defense, and sometimes, compensation for emasculation.
Guns are associated with atrocities because they make them so much easier to perpetrate. Mass atrocities are sometimes mistakenly called "killing sprees." According to criminologists James Allen Fox and Jack Levinthe term "spree killing" identifies events that fall neither into the categories of serial killing nor mass atrocity. Thus the 1959 killings of the Clutter family of four in Holcomb, Kansas by Richard "Dick" Hickock and Perry Smith were described in press accounts as a "killing spree." When the numbers of victims climb beyond the single digits and a political rather than criminal motive is evident then the event is normally called an atrocity. Unless of course one wants to diminish the horror of the event. The term "spree" connotes irresponsibility rather than rational intention, as in a "shopping spree." Curiously, some of the English language news coverage of the atrocity committed by Anders Behring Breivik uses the term "killing spree."
Any gun that doesn't happen to have rifling in the barrel. Shotguns that are designed to not fire solely slugs are typically smoothbore, but compensate for this with a choke to increase accuracy. Most guns before the rifled musket were smoothbore.
A rifle is a long barrelled gun with rifling, where the barrel is grooved and twisted to spin the projectile as it passes through. That's about all you need to describe it.
An assault rifle is a
big black scary tube that makes loud bright flashes menacingly term for a subset of rifles with specific features (pistol grip, grenade launcher, adjustable stock, etc etc.) making them more intended for fighting wars over personal defense or hunting. They are widely used by military and law enforcement forces around the world. They are also often discussed in the debate over gun control (in the USA and Europe at least), but their exact definition is somewhat murky.
The major features of an assault rifle are man-portability, the use of intermediate rifle cartridges (as opposed to handgun cartridges, full power rifle cartridges, or shotgun shells), detachable magazines, being capable to accept a range of modifications and attachments (Picatinny rail), and being capable of full automatic fire. The US military definition of an assault rifle specifies that it must have a selective-fire function (meaning that it can be switched between semi-automatic and burst and/or fully automatic firing modes).
I.e. standard issue rifle. Doesn't necessarily have to be an assault rifle so long as it is a standard for your run of the mill grunt. 
Assault weapons/Black rifles
The Federal Assault Weapons Ban of 1994-2004, applying to civilian gun ownership, defined assault weapons specifically as semi-automatic rifles (i.e. ones which fire one round for each trigger pull) with certain features. Many of the guns subject to the Federal Assault Weapons Ban were semi-automatic rifles which had been adapted for the civilian market from select-fire military service rifles. Since the ban ended, some states (generally more populated and closer to the coast) have taken upon themselves to limit the type of guns one can own, while others (generally rural) have expanded the allowances to own such. 
These guns are fairly prevalent in the USA, due to a range of factors such as lax gun regulation, demand for said guns, gun ban panics (which in of themselves are known to spike gun sales, even conservatives admit this), utility, interchangeability, abundance of parts, and popularity. That said, the AR-15, AK clones, and similar style firearms can be seen in roughly every gun shop, show, private sale, etc. This is especially true for the AR-15, where it and its copies are very common in every state that doesn't have an outright statewide ban on them since it started getting sold to civilians (1964 onward to, as opposed to its original design as service rifle in 1959), because that surely isn't overcompensating for something, like being able to spit expanding copper-coated lead at hogs and small children at over 2000 feet per second. The AR-15 has had a rather bad reputation thanks to it and its clones being frequently used or seen in school shootings and other civilian gun massacres (or at least the very least sensationalized by the media and severity of the shootings, as majority of mass shootings are done by handguns, but usually aren't as severe), before and after the Assault Weapons Ban. Speaking of which, it should be noted that handguns have also been used in a fair amount of these massacres (such as solely in the case of Virginia Tech shooting and in addition to the XM-15 (AR-15 copycat) used in Sandy Hook shooting.)
Light machine guns
These fully-automatic weapons are usually loaded from a belt of cartridges composed of rifle or specialized machine gun rounds. Some machine guns use magazines, especially earlier ones, though they are making a comeback. They are difficult to use because of their size and weight and are generally illegal in the USA for civilian ownership. These differ from heavy machine guns, which are usually too... um... heavy for one person to carry and fire, and must be mounted on a rigid platform or disassembled and carried by multiple people. Since 1986, no newly manufactured machine gun can be registered for private ownership. It is still common for companies to make semi-auto only machine guns to get around this, though.
Typically smaller than an assault rifle (thus being easier to use in close quarters combat situations). Submachine guns (often abbreviated to SMG, because, seriously. Who wants to write that all out every single time?) typically have less range due to a shorter barrel (almost always below 10 inches, compared to rifles which are almost always longer than that) and use of lower-power handgun ammunition. Police tend to favor these weapons The Thompson sub-machine gun (the classic "Tommy Gun" associated with 1920s gangsters and 1940s grunts) is perhaps the most famous example, but the Uzi, MP5, and PPH-41 also fall into this category. Many state police and security forces use submachine guns because of compact format, light weight, ease of use, and risk of over-penetration and collateral damage being reduced. They are regulated under Title II and civilian ownership is banned outright in many states (except for semi-automatic clones, which seem to be accessible in the most stringent of states).
Han Solo's blasterA Mauser C96, an early self-loading pistol. This one comes with a stock that can be attached to the grip and sights that can be calibrated all the way out to over a kilometre.
Handguns are the smallest firearms, use specific ammunition and typically are not made with stocks, though there are some exceptions. They usually have semi-automatic or revolver actions. They also end up in many mass shootings and are responsible for roughly three fourths of annual gun deaths in the United States. What's trendy with handguns these days are polymer frames. These are sometimes met with derision for being "plastic toys", and even caused quite the fuss when they were first introduced in the 1970s and 80s about them supposedly not showing up on metal detectors, despite the need for metal parts when designing a functioning and "safe" (for lack of a better word) gun having not changed with the introduction of polymer-framed guns.
Another double-barrelled shotgun. The barrels on this one have been cut down, something often done by gangsters both back in the day and today.
Shotguns are defined by the type of ammunition they use: the shotgun shell. Shotgun shells typically contain numerous small lead balls (or shot), but can also contain lead slugs only slightly smaller than the shell itself or less-lethal munitions like beanbags and rubber balls. Shotguns typically come in break action (often with multiple barrels in various configurations; the most common number is 2 and the most common configurations are either side-by-side or over-and-under), pump-action, and semi-automatic varieties, but lever-action, full-auto, and revolver shotguns also exist.
A sniper rifle with a largely defensive weapon of hunting rifle. Note the assorted differences.
Sniper rifles are typically rifles equipped with some form of telescopic scope (though in the past, people like Simo Häyhä made do with iron sights). They are usually chambered for full-power rifle cartridges, but some use even bigger rounds like .50 BMG or 12.7x108mm (Russian .50), typically intended for use in heavy machine guns. Pretty much the only difference between a sniper rifle and your usual hunting rifle is quality. Military get rifles decked out with match-grade ammo, precision barrels, good quality scopes, and various other attachments. Hunters and gun enthusiasts get what they can afford. Also what is usually on the other end of the scope. If it's an animal, good news! It's just a hunting rifle. Person? Sniper rifle. Fairly simple, right? Well, sniper rifles are often used by both sides of the law, as well as military forces. A related concept to the sniper rifle is the "designated marksman rifle", or an accurized military rifle, often with select-fire capabilities.
Due to their nature, most people do not have direct experience with guns, only seeing them in movies, television, and more recently, video games. Almost none of these products are meant to entertain portray guns and what they can do accurately, and more often than not show guns doing things that would be unlikely if not completely impossible. These misconceptions tend to color all sides of the gun debate, from gun control advocates who only have the slightest concept about how guns work to those who greatly overestimate their knowledge. While most people are aware that movies are fictional, it is not always easy to tell what is realistic and what is not.
Handguns tend to be portrayed the most inaccurately. The recoil is barely ever shown properly in movies, and characters often handle and shoot them with one hand. While not impossible to do so, it is very hard to shoot accurately and keep control of the weapon with one hand, which is why real trained shooters get into a proper two-handed pistol-shooting stance by reflex. Ammo capacity is also inflated for entertainment purposes, most semi-automatic handguns have between 7-15 shots per magazine, revolvers typically have 6-7 shots and have to be cocked before each shot. Fully automatic assault rifles in particular are portrayed as wildly unrealistic, turning a single person into a killing machine able to mow down perhaps dozens of people without having to reload. In reality, fully-automatic weapons are very hard to control beyond a three-round burst, and even a 30-round magazine will deplete very quickly if fired like they are in movies.
The nuances of ammunition in general are complex. There are many, many different types of ammunition for different guns and purposes, and in some ways the types of ammunition used are more important than the type of gun used. While there are many different kinds of ammunition, all rounds with metal bullets are lethal (and even rubber bullets and beanbags can be lethal). Smaller, "pea-shooter" rounds are often maligned as being barely lethal. The .22LR round is a very common round that is much smaller than most and typically only used for casual, inexpensive shooting and training. The Israeli military tried to claim the .22LR round was "non-lethal" in order to justify shooting stone-wielding protesters with them . The late Ronald Reagan might be surprised to hear that, as a .22LR round would have killed him if they were a little farther away from a hospital.
Oh, and guns are loud. Like, way louder than you're probably imagining them. Too much exposure to guns without ear protection will almost certainly cause hearing problems, and firing in a confined space can quickly leave you temporally deaf, a fact lampshaded by some shows like Archer. "Silencers", or more correctly "suppressors" do exist but are not as effective as shown in movies. In order to make a gun significantly less ear-splitting you have to design it from the ground-up for that purpose, not just screw on an attachment.
Guns able to fire more than one round from a single barrel per trigger pull are considered fully automatic weapons (though techniques and modifications to anything that isn't the upper receiver can bypass that). In the U.S., these are regulated under Title II of the National Firearms Act, which also regulates weapons like "sawed-off" shotguns, "short barreled" rifles and high explosive devices. Civilian ownership of Title II weapons is heavily restricted and limited by a lengthy and expensive permit process along with heavy taxes (such as $200 per weapon manufactured and/or sold). Many states also have additional regulations on Title II weapons, with some, like New York and California, banning their ownership by the general public outright. Some semi-automatic assault weapons like the AR-15 can be converted to fully automatic by changing out a few parts (aka remove the lower receiver, put an new one in that is capable of full-automatic fire); these parts are also regulated under Title II.
Some of the issues pertaining to gun laws, like those within the US, is that they tend to be vague, reactionary, and ineffective. A majority of the laws within the US can be bypassed by someone with an IQ equal or above that of Forrest Gump by either their own skill (i.e. machining) or going to the next state over.
- Bowling for Columbine
- Dick Cheney
- Gun control
- Charlton Heston
- Second Amendment
- National Rifle Association
- Ted Nugent
- A Brief History of the Assault Rifle, The Atlantic
- "Multiple Homicides: Patterns of Serial and Mass Killing." Crime and Justice. Vol. 23 (1998): 407-455]