"Lucifer" generally refers to the Devil/Satan, although the name is not applied to him in the New Testament. Post-New Testament Lucifer is often used as the name of the Devil and often thought as the Devil's name before he fell from Heaven and cast out of Heaven by God himself.
Lucifer is greatly known as the first fallen angel in origin. Lucifer, whose name means "bearer of light" with other variations like "shining one, morning star, light-bringing, bringer of dawn" and associated with the planet Venus. Venus rises in the morning to eclipse all of the stars but the Sun comes to overtake the planets visibility. Lucifer also has been known to take the form of a dragon and serpent among many other things.
War in Heaven
Lucifer was once the most loved Angel by God. God cast Lucifer out of Heaven because he refused to bow to Adam, the first man, out of envy and jealousy that Adam had a soul and was granted power over the earth. Most Christian writings say that the angel rebelled out of pride against God in regard to his judgement over humanity with another variation being that he wanted to be greater than God.
The first "War in Heaven" between angels was led by the Archangel Michael against Lucifer (the dragon, devil, satan). All angels who sided with Lucifer were thrown down to earth later to be called Fallen Angels. This rebellion of angels was before the Fall of Man (later described in the Garden of Eden below). One third of the angels in Heaven were thrown down to earth and lost their grace of God. Among the fallen angels were pagan gods such as Moloch (Molech) and Belial.
"How art thou fallen from heaven
O day-star, son of the morning! (Helel ben Shahar)
How art thou cast down to the ground,
That didst cast lots over the nations!
And thou saidst in thy heart:
'I will ascend into heaven,
Above the stars of God (El)
Will I exalt my throne;
And I will sit upon the mount of meeting,
In the uttermost parts of the north;
I will ascend above the heights of the clouds;
I will be like the Most High (Elyon).'
Yet thou shalt be brought dow to the nether-world,
To the uttermost parts of the pit."
- Isaiah 14:12-15
Lucifer as originally called but later to be called Devil and Satan is referenced in ancient texts such as the Bible, Dante Alighieri's Inferno and John Milton's Paradise Lost.
King of Babylon
Referenced to an ancient "king of Babylon" (not referred to as an angel or a god but a man):
"How you have fallen from heaven, morning star, son of the dawn! You have been cast down to the earth, you who once laid low the nations! You said in your heart, 'I will ascend to the heavens; I will raise my throne above the stars of God; I will sit enthroned on the mount of assembly, on the utmost heights of Mount Zaphon. I will ascend above the tops of the clouds; I will make myself like the Most High.' But you are brought down to the realm of the dead, to the depths of the pit. Those who see you stare at you, they ponder your fate: 'Is this the man who shook the earth and made kingdoms tremble, the man who made the world a wilderness, who overthrew its cities and would not let his captives go home?'"
- Isaiah 14:12-17
In ancient Canaanite mythology, the morning star is referred to as a god, Attar. This god attempted to occupy the throne of Ba'al but could not. He instead descended to rule the underworld, Hell.
Garden of Eden
In the Garden of Eden, the serpent (Satan) tempts Eve to eat from the forbidden fruit, an apple, from the tree of knowledge after God told them not to in Genesis 3:1-14. The result was that Adam and Eve were cast out of the Garden of Eden. The serpent was cursed by God to crawl on his belly for eternity and a forever reminder of the Fall. It is recorded science that around 95 million years ago snakes lost their legs.