Welcome
GURPs in an RPG, role playing game (system), now in it's fourth edition, produced by Steve Jackson Games, I'll leave you to read the official website or the Wikipedia Article to find out more. GURPs Lite (a cut down, but workable system) is available for free from their website, so give it a go.
As a system, GURPs has no linked background or setting (the place where your games will happen, the Serenity RPG is obviously set in that universe). GURPs is generic, this is it's strength and weakness. I like it, especially the emphasis it places on making your characters from their skills.
I plan to add details of some of the plots I've GM'd (Game Mastered, i.e. games I've run). Sadly, time is short at the moment, so I'll get there eventually.
