A great game, and no game has yet touched its fame for science fiction tabletop RPGs. Traveller covered all the "space" things in a detailed setting called the Third Imperium, a unified star community ruled by an Emperor. Traveller spawned an extensive series of supplements, which you can take or leave as needed for the emphasis of your particular game, whether star merchants or space mercenaries or explorer scouts.
Star sector maps are laid down in hexagons, each is a parsec (3.26 light-years), and starships have Jump Drives which can Jump anywhere from 1 to 6 of those parsecs but each Jump takes a week. There is no faster way to get news or goods across the stars, so the political feel resembles 18th-century sailing. The Imperium is held together but its size is causing strain. The Imperium also borders on alien empires which range from coldly neutral to hostile.
The Traveller Starter edition (1982) did some reorganization of the original LBBs (Little Black Books) of 1977 and presents all the tables of the game in a separate booklet, cross-referenced to the pages of the rulebook.
Traveller is "old school" and the best way to learn it is to take its systems one at a time. Randomly generating star-maps is particularly fun, with the main world in each system defined as to size, atmosphere, population, and government with rolls of six-sided dice (only). Nowadays the fans offer web-based computer utilities to instantly generate vast tracts of space, or use the defined Imperium maps already available online. The personal combat system, in basic Traveller, is NOT on a grid-map but more abstractly defined with "bands" similar to a football field! There are also rules for trade (buy low, sell high of course), which is easy to do as you note the characteristics of the world you're going to (Trade Codes like INdustrial or Non-Industrial, Agricultural, etc.) which give bonuses or penalties to the negotiated price of cargo. Nothing is a sure thing, however! The starship combat, in the original version, is not gridded either, but takes note of what you have for weapon turrets and rolls the damage to the other ship and what sub-system was damaged.
Definitely a solid science fiction experience.
[5 of 5 Stars!]