A white person. Derived from the -CRACK- sound of a whip hitting someone in slavery times. Sometimes used as a mild insult from a black person to a white person(1) but sometimes a term of endearment(2). Also see nigga, nigger.
(1)Black Guy: Look at that cracker, tryin to wear his shit like us, we should bust his punk ass!
(2)Black(Or White) Guy: Hey cracka, whassup man?
White Guy: Nothin much. You know, thats fucked up, you can call me a cracka but I cant call you a nigga.