He's under contract for 2013, it's just that his salary is yet to be determined. When you only have so many years of MLB service (unless you sign a deal in that time) you have arbitrational years which essentially allow players to get a "fair" salary comparative to other players their age with similar numbers.
Pence I believe is entering his last year of arbitrational eligibility so unless he and the Giants can agree to a long term deal, they'll either agree to a salary for the upcoming season or if they can't they'll have an arbitrational hearing. Given his previous salary and essentially you're pretty much do for a raise in most scenarios, it's expected it'll be about $13-14 million for a one year deal. When you run out of aribriational years (I think it's up to six years?) then you no longer have that right to "fight" for money on your one year deal.
Teams can also "buy out" arbitration years to avoid this, like the Giants did with Bumgarner (and what they'll do with Posey) by giving them a "normal" contract.