The official currency in Costa Rica is the colón (pronounced 'ko-lo'na') but American currency is accepted almost everywhere in the country.
Even off the beaten path and away from the tourist areas, Costa Ricans are used to getting dollars. The current transfer rate is about ˘520 to $1. As a frame of reference, think of a ˘5000 bill as $10 and a ˘10000 bill at $20. It's good to carry a bit of both currencies, especially if you like to haggle and vendors aren't always as good at math as you might be.