There used to be two currencies used in Cuba, but in 2021 that changed and there is now only one: the Cuban Peso (CUP).
The Cuban government has recently made drastic changes to the USD-CUP exchange rate, so be sure you know what the latest rates are. It can be very helpful to have the assistance of a trusted local planner to make sure you're getting the best rate.
For first time travelers to Cuba and those traveling independently (without the help of a local planner) we recommend exchanging money at a CADECA at the official rate. If you are working with a local Cuban planner, we recommend asking them to provide you with information on the current inflation situation and whether it makes sense to work with an airbnb host or some other trusted local money changer to get the better rate.
It's also a good idea to keep some of your money as USD, since they are almost always gladly accepted if you happen to run out of CUP.
When you do change your money in Cuba, know the agreed upon exchange rate and how much money you should expect to get back. One of the more common scams you are likely to encounter is getting ripped off at the exchange counter. Be sure to always pay attention and don't easily accept whatever is handed to you. Count your money and correct any mistakes at the moment of your transaction.
Cuba is the safest country in the western hemisphere, but you should still keep close track of your money and don't keep it all in the same place. Cubans know that Americans are carrying cash only, which makes you a target for pickpockets. No need to be nervous though, a little pre-trip research and some common sense will go a long way here, as will your money in Cuba.