Classic Traveller, also known as CT, was the earliest edition of the traveller role-playing game as published by Game Designer`s Workshop. It's official setting was the year 1104- of the Third Imperium, thus set shortly before it peaked out and shattered in the Rebellion.
Classic Traveller was the purest form of the game, with a plug and play rules system, where new books introduced more advanced rules for almost every aspect of the game, which could be used to replace the more basic rules from the first books.
The publications were sorted into books, which contained the main rules, supplements, which contained additional information and play-aids and adventures, which contained, well, adventures. There were also board games such as Imperium, Snapshot and Invasion: Earth and the Journal of the Traveller's Aid Society (JTAS), which was the official magazine.
CT was one of the first SciFi RPG's on the market when it came out in 1977. This year, fans are celebrating it's 25th birthday. All of the CT books are recently being republished by the game's head designer Marc W. Miller in larger collected volumes, the last one's are due this year. Well worth checking out.
Links:
A complete list Classic Traveller publications can be found here:
http://www.pemaquidsolutions.com/bibliography/
The homepage of Marc W. Miller is here:
http://www.farfuture.net
A great source of further information, adventures and game aids can be found here:
http://www.downport.com/