The Traveller’s Handbook Lite Edition, RPG Realms, 2002. You got your D20 in my Traveller.
Most people live on a single world and don’t ever go to the stars. A few might travel once or twice on business or for a holiday. Travellers are different. Travellers (by definition) travel. Not all Travellers are adventurers; many are professionals, merchants, freelancers or whatever, who make their living wherever they can find employment.
Player-characters are usually Travellers (though their actual classes and professions vary considerably; they need not be of the Traveller character class). What they have in common is a willingness to confront a few risks and maybe bend some rules to get the job done. Unlike most fantasy adventurers, Travellers tend not to be inexperienced 1st-level characters. A typical Traveller is 26-46 years old, and has been around a bit. Some have done a stint in the military; others are university graduates or have pursued a career. A few have done all these things, and more! Typically, a starting Traveller character will be level 3-7 depending upon his or her age, with skills and experience to show for it.
This makes Travellers handy people to have around, and in a universe where “official” help is weeks away, it is often to Travellers that local organizations will turn when they need a job done or a problem solved.