A Chaos Sorcerer must take at least one spell from the deck of his patron Chaos God, and may choose additional spells from the Dark Magic deck. Beastman Sorcerers may use spells from any Chaos God/s, and from the Dark Magic deck.
Chaos Sorcerers receive a Mark of Chaos. This Mark is unique to their patron Chaos God, and imbues the Sorcerer with a special inherent magic ability, which does not need to be powered with magic cards.
See page 43 of Warhammer Armies: Chaos for more details.
Chaos Sorcerers may wear Chaos Armour without it affecting their ability to cast spells.
Chaos Sorcerers may be bought Chaos Rewards, though Beastman Shamans may not.