Cuba has 2 currencies, CUP(for Cubans) and CUC(for non-Cubans)
1 USD = 1 CUC = 24 CUP
You probably won't have to deal with CUPs, unless a Cuban gives you change back - be sure to check your change. CUC and CUP look VERY similar, so it's easy for Cubans to short-change you if they desire
It is not possible to get CUCs before arriving in Cuba, so you will need to exchange your money upon arrival. You can exchange your money at the airport
ATM machines are available for use for non-US visitors, but the lines are typically very long
Note that US debit cards do not work at Cuban ATM machines so US Citizens will need to exchange all of their money at once
TIP: There is an automatic 10% penalty when you exchange USD to CUC conversions, so it's recommended to exchange your US dollars to Euros before arriving and THEN exchange your Euros to CUC