Here is my take:
Chaos Space Marines: Corrupted marines that worship the chaos gods, fun to make, have awesome stuff like dragons, zombies and dubstep guns
Dark Angels: Deathwing: super elite terminators with nifty special rules. Ravenwing: Semi-elite bikers with awesome gear. Greenwing: Can be like regular marines, but standard of devastation makes them into the perfect sit and shoot marines
Blood Angels: Berzerking space vampires, it's all about the assault squads and sanguinary priests, who give them feel no pain. Their head librarian used to be the most OP unit in 6th edition, now he's kinda meh
Dark Eldar: S&M elves in spaaaaace! Epitome of glass cannon-ness, lots of poison, lances and speed on super fragile guys
Grey Knights: Psyker space marines that specialize on killing Daemons, accompanied by the forces of the inquisition, very versatile.
Imperial Guard: Average joes! As HFY as it gets, they can be deployed in hordes, lots of vehicles or heavy weapons galore, either way, you'll have UNGODLY firepower
Necrons: Space zombie robots from looooong ago, kinda egyptian vibe, great mid range shooting, broken fliers and interesting HQ's
Sisters of Battle: Just don't.
Space Marines: Jack of all trades marines, encourage players to make their own chapters and have fun converting!
Space Wolves: Vikings in spaaaaaceeee! They have awesome stuff like dudes that ride wolves and lone dudes that wreck shit and give you points if they die!
Tyranids: Aliens most horrifying! good at either swarming the enemy or demolishing the field with Godzilla-style super-beasties!
Black Templars: Marines that are angry, very angry. They make mixed squads with marines and scouts, but if you're not careful they will do whatever the fuck they want!
Chaos Daemons:DEMONS, in spaaaaaace! All about the rolls. they all deep strike and have some broken OP units, but others leave much to be desired. Mostly a melee army.