The colon is the Costa Rican unit of currency the exchange varies between 505 and 520 colones per dollar. US currency is accepted almost everywhere in Costa Rica with many but expect to receive your change in colones. To keep things simple it is easiest to equate 500 colones to the dollar, however the Costa Ricans will use the daily exchange rate and pocket calculater to issue correct change.
At one point the Costa Rican banks issued a notice for businesses not to accept US $100 dollar bills due to incidences of forgery but you can change them at banks or currency exchanges. In general you would be hard-pressed to find any place that would be able to make change for it large bills, it is useful to have smaller bills on hand to facilitate getting your change promptly at some of the smaller locations.
ATMs are common throughout Costa Rica, especially in tourist destinations. Still, it is a good idea always to keep some cash on hand. Most ATM's service either the Cirrus (Mastercard) or Plus (Visa) network. A good idea would be to take one of each of these cards and to make sure that each has enough funds to get you out of a jam. Many travelers have observed that Visa is the most common card accepted followed by Mastercard. American Express and even Diners Club are farther behind but are accepted at a significant number of locations.
Word to the Wise
If you are in Costa Rica for an extended period of time paying bills can be a hassle. It would be recommended to set as many of your bills on auto pay before you leave. Write down your checking account number and routing number to pay off any credit card bills while you are gone over the phone (some won't take numbers of other credit/debit card numbers). Online credit card transactions can be a hassle too because many companies will detect that a US card is being used out of the country to order merchandise and red-tag the transaction. If your planning on using your card to send a gift or flowers consider allowing a couple of extra days for the transaction to go through.