Typically the step is a low flat bench, but my gym also has wooden boxes of various heights. I asked about this b/c I read about Russian and Bulgarian coaches who switched over their "clean and jerk" competitors from standard squats to step-ups. I don't know if they still do this, but the coaches talked about how it gave their legs a different sort of look to them and felt it added more mass. One of the guys was doing this exercise with like 315lbs or more.
actually one of the best all around quad exercises there is.. very underrated..