There are different factions among mages. These are groups who have (sort) of the same theories of how magic and reality is supposed to work.
Usually, a character is already on his way to a bigger understanding of reality before truely Awakening. His experiences prior to awakening will determine largely in which faction the character will feel most at home. Someone who has always worked with gizmo's and gadgets and gets them to do funky things, will feel right at home with the Sons of Ether, for example. He of she isn't automatically a Son of Ether, but in the course of the game, she will likely be tutored by one and join them.
Picking a faction at character creation is just a shortcut for trying to convey what kind of magic the character will be able to do. If someone would tell me "I want to be a Verbena", this gives me a rough picture in very little words. If you don't know the factions, you could also say "I want to play a witch-type mage, someone who is dealing with runes, bones, curses and lots of blood." This gives me exactly the same information.
So, the factions just give you a starting point, they show you what kind of characters would be possible. But it's also completely viable to think of a kind of magic, and pick a faction later in-game. If you'd like a run-down of the factions, I'll post it here, but you could also just think of /any/ kind of extraordinary character and handle the factions in-game. Every person who is capable of doing the supernatural could be a Mage, faction can always be pegged on later.
Did this make sense? I hope so. Please let me know if some things aren't clear yet.
Fanog