Within the majority of Christian denominations, the term fallen angel is a direct reference to an angel or spiritual being that has been thrown out, banished, or exiled from Heaven. It is usually the direct hand of God that expels these angels from his kingdom, however occasionally other powerful righteous angels conduct the task, such as the mighty archangel Michael. With regard to the expulsion itself, it is usually the result of an angel or group of angels rebelling or disobeying God himself. Through vanity, wickedness, or other sinful deeds, these angels corrupt themselves, and are cast out of the pure glory of God, which in itself is whole and incorruptible. Most of the extremely detailed literary works that illustrate these disturbances or angelic rebellions, can be found in noncanonical Biblical scripture, or in works of fiction that pull from the Bible. Many monks and scholars have contributed much to the intriguing concept of fallen angels.
Many of you are undoubtedly aware of the War in Heaven that took place in the Bible, and the most famous fallen angel of them all, which was Satan or Lucifer. Outside of Judaic scripture, Lucifer is usually called Satan, as is the case within most denominations of Christianity. He is also called the Day Star or Morning Star, which is derived from the fact that he lit up the sky when he was cast out of heaven by Michael. The first mention of Lucifer was in the Book of Isaiah, Chapter 14, verses 3 through 20. This portion of scripture discusses a being referred to as the Morning Star, who was cast out of Heaven. The name Lucifer is simply the Latin name given to this creature, which obviously went on to have a profound character attached to it, despite that not being the case early on at it's inception. What's also interesting is that in Greek translations of the Bible, Satan is called Eosphoros, which translated means the Light Bearer or Holder of Light.
There are several highly detailed hypotheses and myths that go on to explain the causes of the fall of angels. Most of them are based on the possession of free will by the angels, which obviously springs forth individual feelings, desires, pride, and most importantly the potential to misunderstand or misinterpret the will of God. As we all know, Lucifer was punished for all of the above, desire, independence from God's will, and pride, the later of which drove him to think that he possessed a higher truth above God himself. Due to these infractions Lucifer supposedly lost his wings during his expulsion, and is now forced to roam the earth until Judgement Day, after which he will be exiled to the pits of hell for all of time. This epic battle is foretold in the New Testament Book of Revelation, in which Lucifer is named as the great dragon or the ancient serpent who deceived Eve, and who will ultimately be destroyed once and for all.
With regard to the number of fallen angels, there are literally hundreds of well known beings who were cast out of heaven alongside Lucifer. Most of these fallen angels are not referred to by name within Jewish, Christian, or Islamic scripture, they only exist within noncanonical scripture. To name some of the more common fallen angels, they are Phenex, Rosier, Samyaza, Sariel, Abezethibou, Adirael, Baraqiel, Batariel, Bezaliel, Chazaqiel, and many others. Most of these fallen angels had specific duties ordained to them when they were servants of God, however they were reassigned different tasks once under the command of Satan. Some of them roam through the bowels of Hell tormenting the lost souls of mankind, while others roam the earth and wreak havoc amongst the living. Some are responsible for instilling certain feelings within human beings, such as lust, rage, pride, envy, boastfulness, worry, fear, and other heavy sinful burdens.