Player Character Races (DMG)

Many factors affect a character's background. Two of the most important are his race and his character class (see Chapter 3 , "Player Character Classes"). In a sense, a character's class is his profession. Some characters are fighters, some are mages, some are clerics, and so on. A character's race affects which character classes are available to him. Only humans have unlimited class options. All non-human races are limited to some extent. There are two reasons for this:
First, the restrictions are intended to channel players into careers that make sense for the various races. Dwarves are, to a certain degree, anti-magical and incapable of shaping magical energy—they can't be wizards. Halflings, despite their ties to nature, lack the devotion and physical will to be druids. Similar situations exist for the other demihuman races.
Second, the demihuman races have advantages that are not available to humans. Flexibility, the ability to choose from among all the classes, is one of very few human advantages.