A sailor is someone who works on a boat or ship — either for fun, a job, or while serving in the navy. If you want to be a sailor, it's good to start by learning to swim.
If you work on a cargo ship or help to pilot a schooner in the summer, you can call yourself a sailor. You're also a sailor if you learn how to sail a small boat around a lake. Even though the boats that sailors work on don't necessarily have sails, that's where the word comes from — the verb sail. Before sailor was coined in the 15th century, people used mariner and seaman.