Often interchangeable with Powergamer, only a rollplayer does not not always have to be one. For example, someone could want to try out homebrew feats for their fighter, and has a clusterfuck of stats and data, but it has nothing to do with powergaming. Merely, this is how he games. This however, is rare, and the only ever spotted rollplayer that wasn't also a powergamer is Jim Profit. Who, half the time, will shut up if you just give him some poisons and a hug.
Many of these kinds of players enjoy the game itself: the rules, the statistics, the gameplay, and the interactions between far-flung mechanics and vastly disparate class builds.
Typically rollplayers are better suited for games of medium rule set. Something not arbitrary enough where it does not matter, (i.e. World of Darkness) but not so rules heavy that any alteration can slowdown a game by decades (i.e. GURPS).
It should not shock you that many rollplayers have vacated the tabletop space in exchange for vidjageim arpeegees, especially super-deep Rogue-likes. This is partially the cause of rollplaying being conflated with powergaming: hardcore powergamers stuck to the games they were good at, while some earnest rollplayers will jump from platform to platform, exploring game designs from angles that don't always need to include goofy humans.