Sails are large pieces of material attached to the mast of a ship. The wind blows against the sails and pushes the ship along.

You say a ship sails when it moves over the sea.

If you sail a boat or if a boat sails, it moves across water using its sails.

The sails on a windmill are the long flat parts that are turned by the wind.

If a person or thing sails somewhere, they move there smoothly and fairly quickly.

When a ship sets sail, it leaves a port.

If you cross the sea under sail, you cross it in a ship that has sails rather than an engine.