In Cuba there are two currencies: the official currency of Cuba: Cuban Peso or Moneda Nacional,and the tourist currency: Convertible Peso or CUC. Most goods and services can be bought exclusively by tourist currency ConvertiblePeso, while the Cuban Pesos or Moneda Nacional, can be used in public transport, in the stalls along the road, or in other shops away from tourist destinations.
It is also suggested to convert few euro/dollar into Cuban Peso Moneda Nacional, useful to catch the bus, buy food and drinks along the way or some fruit from the farmers.
The Moneda Nacional can not be converted-back into the original currency, while the Convertible Peso or CUC can be converted-back in the airport. It is recommended to bring a credit card for emergencies but not issued by Institutes U.S. or connected to U.S. banks.