Onboard sound cards aren't as good as the high-end add-in cards like the SoundBlaster Live! and Turtle Beach Santa Cruz, but if you're just doing basic stuff and no real sound things, ie, heavy music playing, movies, games, then onboard will serve you well.
An add-in card will give you things like EAX, A3D, and other perks, like better sound quality. The onboard sound is usually about the equivalent of a $20 PCI sound card, if that.