T20 is a conversion of the Traveller Science-Fiction RPG to the d20 System. Traveller was first published in 1977, and has gone through many incarnations since then. With each new edition, the background became deeper and richer.
While various Traveller products use other rules systems, the background, library data and setting are all compatible with T20. Conversion to these rules is quick and easy, since most Traveller products are rules-light. Similarly, ideas and even whole scenarios can be “borrowed” from other games, movies or novels without difficulty. Traveller is not a thing of rules; it is a setting and a Space-Opera-meets-Hard-SF style of gaming.
And it’s fun.
Welcome to T20.