Phobia

A phobia is defined as an "irrational fear", in other words, this refers to a fear which is not based in any geniune danger, or in which the danger is blown out of proportion. For instance, it may be perfectly rational to feel fearful if you're being chased by an axe-wielding psychopath bent on killing you, but not so much so when you're on a plane given how unlikely it is for plane crashes to occur. In order to protect itself, when the body feels in danger, it triggers the fight-or-flight reflex in order to help it fight off or escape a threat. However, when this reflex is triggered unnecessarily, it can produce traumatic symptoms.

Common phobias
The Phobia List has an extensive list of phobias, although it's unclear which are genuine and which are just the result of the compiler looking up random terms, converting them into Greek and sticking "phobia" on the end to make the word.
 * Acrophobia — Fear of heights.
 * Agoraphobia — Fear of being outside a small, comfortable and familiar space. Often mislabelled as "fear of the outdoors", although this is a fair description of most cases.
 * Arachnophobia — Fear of spiders. This is very common, despite the general harmlessness of most house spiders.
 * Aviatophobia — Fear of flying.
 * Dentophobia — Fear of going to the dentist (though there could be a good reason for this: dental woo).
 * Chemophobia — Fear of chemicals.
 * Haphophobia — Fear of being touched. This may stem from child sexual abuse.
 * Pithecophobia — Fear of apes.
 * Paraskavedekatriaphobia — fear of Friday the thirteenth, a.k.a. friggatriskaidekaphobia.
 * Triskaidekaphobia — Fear of the number 13.

The names given to some phobias would appear to be self-defeating. For example, calling the fear of long words "Hippopotomonstrosesquipedaliophobia" is about as cruel as putting 3 t's in "stutter" and an s in lisp.

Non clinical use
The suffix -phobia is often added to terms to mean "distrust" or "hatred". This is seen in homophobia and xenophobia and are used pejoratively. These are not real phobias in the sense that they are irrational fears, they are more political or social stances that are labelled as fears. It would be rather cruel and tactless to deride someone who genuinely had a phobia of gay people.

Related topics

 * Mania: Whereas a phobia is an uncontrollable fear and/or revulsion, a mania is an uncontrollable desire to perform a certain action, and/or an abnormally strong interest in something. Cynomania is an unnatural obsession with dogs; cynophobia is an unnatural fear of dogs.
 * Philia: An unusual love of something. Francophiles love the French; Francophobes fear and/or detest them.
 * Personality Disorder: Generally, a personality disorder is a collection of stereotypical traits and actions that diminish a person's ability to function effectively in society. Many people with personality disorders have one or more (usually many more) phobias, manias and/or philias.