Fallen angels are angels which have fallen. (Well, duh!) Unfortunately, this often ends with their penis accidently slipping into a mortal girl's vagina.
Fallen angels are a group of celestial beings who "fell" (ahem) to the earth while performing their task of watching the innocent teenage daughters of Adam. This descent most likely occured as the result of literally slipping on body oil, which these angels liked to rub on themselves and their well-muscled angelic bodies. (See also: What to do with an erection in Heaven.) Did I mention it was extremely boring in heaven?
The story of the fallen angels began with their angelic leader Michael, who was sent by God to earth and told to guard the new creation. Michael became the guardian angel of God's earthly kingdom, and assigned a crack team of cute male angels to help him in his task.
Originally these angels had gay male sex with each other, but soon realized this sucked ass. In time, they began to look more and more to the charming, milky-skinned girl-flesh which walked upon the earth, and this soon led to problems.
Fallen Angels in the Bible
Genesis 6:4 "And there were some extremely aroused sons-of-bitches in those days, and also afterwards, when the sons of God came in unto the daughters of man, and sayeth unto them, 'Hey little girl, you want some candy?"
Enoch 7:1 "It happened after the sons of men had multiplied in those days, that daughters were born unto them, innocent and naive. And when the angels, the sons of heaven, saw them, they became enamoured of them, saying, "Holy fuck, are you getting all of this? He must want us to hit it..."
Following the astute observation that the women on earth were somewhat pleasing to the eye, the angels were then said to have 'descended' to the earth, in order to get their angelic groove on. Afterwards, they are said to have made swords, taught magic, increased fornication, supposedly invented peanut butter, started the Kentucky Fried Chicken corporation, then retired to sit on the couch and play video games. To house these gaming systems, they built castles, the height of which was 3000 ells, whatever the hell that is. They then wrote the script for Final Fantasy IV, (unfortunately at the time this was on RM2K and thus only a demo), taught women to wear jewelry and makeup, forced them to shave and wear deodorant, and incited them to random acts of lesbianism. They then invented birth control, alcohol, and recreational drugs.
What happened then
Of course, this kind of world could not be allowed. Recovered Gnostic texts recovered at Qumran (see also: Dead Sea Scrolls) have shed further light on the subject, while at the same time alternately suggesting that the angels were not completely at fault, but instead may have been lured to the earth by the women they courted. (See figure 1A.)
According to these texts, the angels were seduced by the daughters of men to their dwellings, and asked to bring the above items (swords, magic, birth control) with them. Thinking they were only meeting to talk, they were instead met by that douchbag Chris Hansen and promptly arrested by Moses and Mahaleel. (That's 2 e's, 2 l's, one h.)