Satan began as Lucifer, angel of the Lord. When Jehovah made Adam and Eve and kept them in the garden, blind and unknowing, it was Lucifer who freed them. The apple was a gift and Lucifer was the one to give it. He gave humans the choice - blind obedience or knowledge? Humans chose knowledge and Jehovah punished them for it. He also punished Lucifer for it, casting him out of Heaven and into Hell. From there, he became Satan, King of Hell, Prince of this World. Jehovah created his own rival and counterpart by giving Him this power.
Satan has two facets, Satan and Lucifer. Satan represents base, carnal desire, indulgence and sin. He is self confidence and pride and retribution against your enemies. He is a dark, intense presence. He champions change and chaos, in such that chaos represents change itself. I believe Him to have chthonic elements as He rules over the souls in Hell. He walks hand in hand with Death and sometimes they share the same face. Satan asks, "Wouldst thou like the taste of butter?" And the Satanist answers, "YES!"
Lucifer represents enlightenment and the quest for knowledge. He is the great muse of civilization, bringing humanity wisdom and art. He is a force of creativity and of exploration of the unknown without fear. Lucifer encourages us to question everything and to think for ourselves. He is the Light Bringer and the Morning Star, shining and bright.
One core tenet of my Satanic philosophy is the idea of choice. We always have a choice and we always have the freedom to choose. Nothing is considered forbidden to a Satanist. Everything is permitted, though not everything is beneficial. Especially things that are considered evil or taboo, the Satanist is encouraged to question why that is and come to their own conclusions on them, rather than simply accepting things as they're told. There is no point in feeling shame about one's desires, especially when that shame is driven by our Christian-centric society.