A person that makes shit happen. Without additional context, "baller" typically is assumed to imply demonstrated ability in generating prodigious amounts of cash money, and/or a proficiency with creating frequent and mind-blowing sexual opportunities (note the
non-gender specific reference, implying that ballers can be men or women, gay or straight -- it is a matter of perspective).
In modern usage, baller does not refer to sports ability -- although the term is likely to have originated with basketball. Still, the reference to "game" is important to the definition of a baller. A baller must
have game. A mad baller must have
mad game. (Here, game is referenced according to the current definition (which pretty can mean pretty much anything, but generally refers to business, social relationships, or desired activities in general).
See also: Mad Baller; Game; Pimp; Rock Star