On the island, the official currency is the Barbados Dollar, which is tied to the US Dollar at a rate of US $1 = BDs $1.98. What this means in reality is 1 US dollar is 2 Barbados dollars, and the two currencies are freely mixed. Avoid carrying large denominations of US dollars, though (ie 100 USD bills), as these are often not accepted in smaller shops and restaurants.
ATM / Cash machines
These are widely available throughout the island, and provide one of the easiest ways to get local currency. Major cards such as Visa are accepted.