An eclipse occurs when Earth, the Moon and the Sun are in a direct line. A lunar eclipse is when the Moon passes through Earth's shadow, and the Moon becomes relatively dark in the sky. A solar eclipse is when Earth passes through the Moon's shadow, and daylight is partially or totally (totality) obscured. During a total solar eclipse the Moon's visible disk covers the Sun completely; day in the path of totality becomes dark, with twilight visible in all directions. Total solar eclipses travel in narrow paths; totality never lasts more than a few minutes. On average, there are about seven total lunar eclipses and fifteen solar eclipses (total and partial combined) per decade.
Neither Earth's orbit round the Sun, nor the Moon's orbit around Earth, are perfectly circular, notwithstanding your fundamentalist cousin's Facebook rant about how life couldn't exist if our orbit were .002% further in or out, or that God placed the Moon at just the right distance to exactly cover the Sun during an eclipse. At apogee the Moon has an angular diameter of 29.3 minutes — at perigee, 34.1'. At Earth's aphelion, the Sun has an angular diameter of 31.6' — at perihelion, 32.7'.
Here's a sort of ASCII-graphics visual aid:
29 30 31 32 33 34 minutes of arc
For almost half its orbit, toward the apogee end, the Moon is apparently smaller than the Sun. Under those conditions, a solar eclipse can never be total, and instead is, at most, annular—meaning a ring of light is seen.
During a total lunar eclipse, the moon appears to don a reddish color instead of black. This is caused by Earth's atmosphere refracting off the sunlight, scattering wavelengths so that it appears reddish.
Blood Moons are associated with Biblical prophecies, foreboding to the apocalypse or something. Christian ministers think that the ongoing tetrad (four consecutive lunar eclipses without partial eclipses) is a sign that the end times are coming. Uh oh, another end of the world prediction to add to the list of doom.
A very popular premodern explanation was that the Sun or the Moon was being eaten by some celestial monster.
- Scandinavia: two wolves  
- The Mayas: a snake
- Some North American First Nations people: a bear
- India: Rahu, a demon
- China: a dragon
- Korea: fire dogs
- Vietnam: a frog
- The Philippines: a dragonlike snake 
So much for revealed wisdom, eh?
An alternative to that was the sorcery theory. The Roman historian Plutarch stated about a certain Aglaonike that she was "thoroughly acquainted with the periods of the full moon when it is subject to eclipse, and, knowing beforehand the time when the moon was due to be overtaken by the earth's shadow, imposed upon the women, and made them all believe that she was drawing down the moon."
The Bible claims that when Jesus died, the Earth went dark, which some take to mean there was an eclipse. Of course, an eclipse could not have occurred at the time of his reported death, and totality wouldn't have lasted anywhere near the three hours claimed in Luke.
The International Flat Earth society believes that the sun and moon are only a few miles across and are only a couple hundred miles overhead. Furthermore, they believe that neither the sun nor the moon ever actually dips below the horizon -- the apparent rising and setting are due to tricks of perspective. Lunar eclipses, in their model, are caused by an unseen dark body occasionally passing in front of the moon.
- Why does the moon look red during a total lunar eclipse?
- According to Joel 2:31 "The sun will be turned into darkness, and the moon into blood, before the great and terrible day of Yahweh comes."
- John Hagee wrote a book about this
- Tim McHyde. "4 Blood Moons of 2014-2015? Mark Biltz Theory Debunked".
- Demons Eat the Sun: Inside Ancient Solar Eclipse Myths - NBC News
- A 360-degree illustration examines the strangest solar eclipse myths throughout history - Washington Post
- Solar Eclipse Myths From Around the World
- Eclipse: Bakunawa eats the sun behind a curtain of clouds | News | GMA News Online
- Plutarch, Conjugalia Praecepta, section 48