A voyager is someone who goes on a long trip, especially if he travels in a ship. Historically, voyagers have often been explorers.
A voyage is a long trip to a faraway land, and people who go on voyages are called voyagers. You're most likely to find this word describing sea travelers, like the explorers Christopher Columbus and Ferdinand Magellan. Because of its association with long, uncertain journeys, the word voyager has often been used for naming spaceships in science fiction and real life. Voyager has a Latin root, viaticum, which means "a journey."