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. While referees were encouraged to use the parts they wanted and change what they did not like, the wealth of background material made the CT setting ubiquitous.
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, during 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 rule systems to handle playing of the game: while characters can usually move from system to system without much effort, other creations such as ship designs will not, though deck plans (often loosely based on the underlying ship designs) are often used across versions.
Traveller aka T1, Classic Traveller or CT
1977-1986 Original Rules. Set in the years 1100-1116 Imperial, around the time of the Fifth Frontier War.
MegaTraveller aka T2 or MT
1986-1991 Task-based system. Set in the years 1116-1124 Imperial, during the time of the Rebellion.
Traveller: The New Era aka T3 or TNE
1992-1995 Twilight2000 rules. Set after the year 1200 Imperial, post Virus timeline.
Marc Miller's Traveller aka T4
1996-1998 Set just after Year Zero, in the early days of the Third Imperium.
GURPS Traveller aka GT
1998-2004 GURPS 3rd Edition Rules. Set in the years 1116-1124 Imperial, in an alternate timeline which differs principally in that the Rebellion of 1116 never happened.
Traveller 20 aka T20
2002- D20 Rules. Set around the year 1000 Imperial, about 100 years before Classic Traveller.
GURPS Traveller: Interstellar Wars
2006- GURPS 4th Edition Rules. Set in 2170 A.D., almost 3500 years before CT, in the time of the Nth Interstellar wars.
Traveller: The New Era: 1248
2006- A rules system free update to The New Era, including the founding of the Fourth Imperium.
Traveller5 aka T5
2007 (proposed). Set in the very early Third Imperium (year 200).
Traveller Hero
2006- A version of the Travller using the Hero Games Star Hero rules and covering several previous milieus including Milieu 0, the T20 milieu 1000, and the New Era.
Traveller: 2300 aka 2300 AD
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 2300 AD. While the systems involved could be played with the traveller rules, the differences are significant.