A sail is the big piece of cloth attached to a mast that propels a sailboat through the water by catching the wind. Part of learning to sail a boat is learning how to use the sails.
Most sailboats have two sails, the mainsail and the jib or headsail on the front of the boat. When you sail, you pilot the boat, or simply ride on it — you can also say you go for a sail. Additionally, the verb sail is used when something seems to float on the air: "The kite sails high in the blue sky." You "sail through" something when it's very easy and quick.