Show photos of sailors, and read books such as "Little Sailor Boy" and "Life at Sea" to help explain what a sailor is.
List and explain the reasons a person might become a sailor. The one students will probably be the most familiar with is that some sailors are in the Marines and live on boats to help protect their country. However, a sailor might also work on a cruise ship or work on a racing or trading ship. Explain that sailors work and live on those ships, at least temporarily.
Talk about the day to day life of sailors and how it differs from their own lives. Discuss how a sailor must have a job on a boat, whether it is driving the boat, cooking for the other sailors, cleaning the boat or running the rigging that makes the boat go.
Take a trip, if possible, so students can see sailors in action. Some suggestions of places to take a trip, depending on your location, would be a local marina, a museum, or an activity center like a science museum, that simulates being on a boat.