The most important thing to any player is their character. The DM tends to have little say over how a character will act or how it will interact with others, but the DM does have a significant say about the alternatives that a character has when advancing or creating a character. In this chapter are rules on how to develop different sub-races of the standard races found in the Player's Handbook. I mean, let's face it, the world wouldn't be the same without Drow to stir things up. The guide also provides rules on how to include non-standard races such as Centaurs, Ogres, Unicorns, and Giants as possible races in your campaign, and even gives all the rules needed to create completely new races.
Races are only a portion, and a small one at that, of a character's profile.
As a DM is developing a world they often develop groups and organizations within their world such as the Harpers in the Forgotten Realms campaign setting. These groups are often comprised of individuals of different races and backgrounds. Third Edition provides a new type of character class to cover those that belong to such groups. These are called Prestige Classes. Several fully-developed prestige classes are provided as examples of possible classes for your players, and possible classes for your villains.