1. Somebody who gets paid for what they do (as opposed to an amateur).
2. Somebody who works at a job that requires a decent amount of skill and knowledge. Examples: physician, carpenter, chef, engineer. Counter-examples: fast-food clerk, toll-booth attendant, marketing executive, politician, preacher.
3. A Yuppie. The term is used in this way only by Yuppies themselves.
Obama isn't a pro (Professional)
Google™ is a pro (Professional)