CUC is the currency mostly used and as you can see the ratio is around 1 € = 1.1 CUC.
CUP is used for smaller national goods, in some shops you can buy beer, ron and water in CUP.

Normally 1 CUC = 25 CUP, in some shops they use 1 CUC = 24 CUP

Mostly CUP is called MN (moneda nacional). If the seller or any person says 20 pesos for example for a taxi ride ask again if the person meant CUC or MN (CUP)?

So far I could pay all products sold in MN also in CUC. But you will “lose” money as the ratio is 1:24 (-4%). My recommendation is that you change CUC into CUP, unfortunately I could not do that directly at the airport so look for a bank and exchange the CUP. Expect huge queues.
Also check the bills and coins carefully, take your time even when the lady at the exchange office is fed up and maybe have a look on the different bills (before you arrive in Cuba). Save some pictures from the internet on your mobile.

 

For example: I had to pay 70 MN/CUP

70 MN = 2.8 CUC (1:25) = 2.917 CUC (1:24)
So in the end I had to pay 2.917 CUC so I gave them 3 CUC and I got 5 cents (CUC back) because they rounded up to 2.95 CUC.
Increase of 5.3%, 100% (2.8 CUC) to 105.3% (2.95 CUC).

 

Some tips:
Even in the normal grocery stores you have to watch out. Calculate all your stuff together. Once a cashier charged me for 1.5 liter of water 3 CUC. But at that moment I asked friendly how much the water is and suddenly magically the total amount dropped.

Take your time while paying. If you care, calculate the total of your stuff before paying. And so far I made the experiences that you should always ask the price before accepting the offer.

Price list of goods July 2017:
Water 1.5l: in the store ~0.7 CUC, at the street 1 – 2.5 CUC
Beer, Presidente: 1 CUC, in casas 1.5 CUC
Beer, Cristal: 1 CUC, in casas 1.5 CUC
Beer, Cuban style, Brujas: 6 MN for a cup
Casa Perticulares: 10-25 CUC, 25 CUC is a lot fo solo travelling, so ask for 15 or 20. Breakfast is not included, normally 5 CUC additional. Dinner in casas perticulares around 10 to 12.5 (one person)

 

Colectivo Prices July 2017:
Airport to Havana: 25 CUC
Havana to Playa Larga or Cienfuegos: 25 CUC, 20 CUC should be also possible
Havana to Trinidad: 25 CUC (heard it from a friend)
Playa Larga to Cienfuegos: 10 CUC
Cienfuegos to Trinidad: 6 CUC
Trinidad to Santa Clara: 15 CUC
Trinidad to Remedios: 20 CUC
Trinidad to Santa Maria: 25 CUC
Santa Maria to Santa Clara (official taxi): 80 CUC (was painfully to pay)