To me a Roll-player is someone where the rules/mechanics of the game are quite important to them, to some more important than the actual story you are trying to tell. A role-player is someone who is more interested in the story of the RPG & doesn't get bogged down in the rules.
For example, several years ago someone I used to RP with was always tweaking the rules of a game to get them more perfect, especially when it came to combat. He was forever trying to come up with & modify the combat rules to make them more realistic when it came to gun combat. Another example of a roll-player is what people call a Rules-Lawyer