There are two currencies used in Cuba. As a foreigner, you will receive Pesos Convertabile, or CUC. One CUC is worth around $1.08 US, though rates are subject to change. The other currency is the local Cuban Peso, or CUP. These are used in a limited number of circumstances and are harder to obtain. They are necessary, however, when paying for bus fare and in certain shops.
Exchangeable currencies in Cuba include Canadian Dollars, Euros, British Pounds and US Dollars. Note that a 10% fee is charged when exchanging US Dollars, so you may want to change them to another currency before your trip. Make sure the notes you are about to exchange are clean, crisp and relatively new. You will not be able to exchange coins. Most places will need to see your passport as well, so be sure to have it with you.
You can exchange your money at a CADECA, or exchange bureau. CADECAs are located at the airport and throughout the country. Know that the exchange rates at the airport are particularly unfavorable to the traveler.
Banks, including the Banco Financiero Internacional and the Banco Metropolitano, will exchange currency. You can also get a cash advance using a non-US credit card. Banks an 11% fee for the transaction.
ATMs are not widespread in Cuba, even in Havana. If you can track one down, you can obtain CUCs using a non-US VISA credit or debit card.