Petty theft is a misdemeanor.
Petty theft when you have a prior conviction for theft MAY be charged as a felony.
The primary difference is the amount of jail time, though you may not actually serve any time.
The worst of it may be the fact you have a felony on your record because it will follow you a long time.
Expect probation, fines, and community service hours.... but jail time isn't out of the question.