There is a lot to say about wifi on Cuba. First, you can access the Internet via the 3G-network, but it is expensive.
Wifi is only allowed in hotels and on hotspots in the major cities. In these wifi-hotspots you will find a lot of people with the mobile phones. To access the Internet costs approximately $2 per hour, which is a lot of money fpr the average cuban.
To be able to use wifi you need to buy a card from ESTCA, the Cuban Telephone Company. You can buy these cards in most hotels