In Cuba there are two currency systems: CUP (the local currency) and CUC (which is always set to the value of USD, that is 1 CUC = 1 USD). Local people earn their salary in CUP, whereas everything related to tourism is paid in CUC. It’s always worth changing part of our money to CUP. In Havana in the local buses you can pay in CUP and in the local food stalls or windows they also charge you in CUP, and if you try to pay in CUC, they will overcharge you probably.
In all cities you will find a CADECA, which are foreign exchange offices, where you can change money. The rates are usually the same in all places, however, we suggest you not change money on the street, where they will deceive you for sure.