Liev Schreiber who plays Victor Creed in X-Men Origins: Wolverine, is indeed essaying the role of Sabretooth (as depicted in X-Men).
Played by Tyler Mane, he was a henchman of Magneto in the first X-Men film and X-Men: The Official Game. Liev Schreiber plays him in the 2009 film X-Men Origins: Wolverine.
Since, your awareness of the comics universe is limited I'll stick to the movie for my explanation. To begin with, he is kind of an arch enemy to the Wolverine. Being his half-brother, he craves for his attention (in a negative way). And at the same time constantly tries (and succeeds) to physically better him to prove that he is more powerful.
Secondly, he only sides with Stryker (in X-Men Origins) as Stryker gives him the opportunity to hunt down Wolverine and make his life miserable. Also he expects that Stryker will transform him into Weapon XI aka Deadpool. But unfortunately, in X-Men he has been depicted as a mindless, growling beast.