What is Traveller?
There are many answers to this question, all of which are both true and false, depending on your viewpoint, and none of which are complete, even in combination.
Traveller is a role-playing system. Traveller was one of the earliest role-playing systems to divorce the concept of skills from that of occupation or class. Characters did not enter the campaign young and untrained; rather, they had careers during which character development occurred. This was generally perceived as giving the game a more "three-dimensional" feel. While there were some assumptions about the campaign world, an interstellar setting called "the Spinward Marches", little actual information was initially provided, leaving the campaign world to the referee's fertile imagination. The system wasn't static, though; the release of supplements and of subsequent editions of the system, brought more detailed character generation, task systems, rules for skill improvement, and additional skills and rules for them. But Traveller isn't a role-playing system, because you can play GURPS Traveller, which is a different system entirely...
Traveller is a role-playing setting. Traveller eventually came to describe an interstellar community of sorts, focused on a "Third Imperium", of which the original Spinward Marches was merely a small frontier area. Published and well-regarded science fiction was a major source of inspiration for aliens and their societies, and for various aspects of technology. A broad history was mapped out, and cultural differences were developed and illustrated. This led to a background in many ways richer than that of previous role-playing settings, yet without significantly limiting the referee. Again, the setting wasn't static; every release of Traveller or Traveller supplements brought new information to light. But Traveller isn't a role-playing setting, because you can play Traveller in a setting that is unrecognizable in comparison to what's in published material...
Traveller is a continuing story. Or at least it can be. As with any role-playing, it can be an opportunity to develop characters, their likes, dislikes, habits, idiosyncrasies, motivations, and so on. The events of any particular session can be part of a grand story arc throughout the campaign. Or not, if the players and referee choose not to play that way.
Ultimately, Traveller is what you, the referee, and you, the player, want it to be. Which is true of any role-playing game, but not very helpful in defining Traveller. We apologize for that, and invite you to share your own answers to the question "What is Traveller?" on our Feedback page.