Anyone travelling to Cuba must possess:
The above permits you to enjoy four weeks in Cuba with no qualms. If you require longer you can easily extend for another four weeks at the immigration offices in most of the major cities.
You will need to pay a departure tax of 25 Convertible Pesos (CUCs) at the airport so remember to save enough notes.
If you're a US citizen wanting to travel to Cuba you should check with the US government to find out the current licencing restrictions. It is definitely possible to travel to Cuba, but you may have to go through a lengthy application process to do so. Entry requirements do change and so it's strongly advised to check with the department of foreign affairs for the latest update.