Versions of Traveller
'Traveller' was published in various editions. Some games tied directly into the RPG system, others were more loosely based on systems from the Traveller Universe. Traveller was orginally designed as a science fiction RPG (one of if not the first), and the original setting was just one the designers favoured. Referee's were encouraged to use what they wanted, and change what they did not like, the wealth of background material made the CT setting ubiquous.
Major Versions
Each of the versions of traveller used a slightly different setting from the traveller universe. Classic Traveller takes place during the third imperium, Megatraveller during the rebellion. TNE takes place after a computer virus ended the rebellion period and a time of exploration/reconstruction. GT takes place in an alternate timeline where events from MT and TNE never occurred. T4, T5 and Other versions of Traveller take place in the early days of the third imperium. Each system also devised new systems to handle playing of the game, while characters can usually move from system to system without much effort, items such as ship designs will not, though use of deck plans often loosely based on the underlying ship designs, are often used across versions.
Traveller aka T1, Classic Traveller or CT
1977-1986
MegaTraveller aka T2 or MT
1986-1991
Traveller: The New Era aka T3 or TNE
1992-1995
Marc Miller's Traveller aka T4
1996-1998
GURPS Traveller aka GT
Traveller 20 aka T20
Traveller5 aka T5
Traveller: 2300 aka 2300AD
This is not actually a part of the Traveller universe. The system was likely lauched with the Traveller name to bring recognition to the line, it is more of a sequel to GDW's Twilight:2000 game system. The name was eventually changed to 2300AD. While the systems involved could be played with the traveller rules, the differences are significant.