A system quarterback
is one who is perceived to be successful but not extraordinarily talented. The term was originally meant to diminish the inflated statistics of players who run spread offenses.
Essentially, it meant the players had strong numbers because they ran an effective offensive "system" rather than being extraordinarily talented.
The implication is that they can win when surrounded by strong talent, but were less of a prospect at the professional level. It is becoming less of a negative tag, as more teams use offensive systems rather than relying on a star.
The opposite of a system quarterback would be a John Elway or Dan Marino who could win a game back the strength of their arm - with or without a strong system
Examples of system quarterbacks are 1989 Heisman Trophy winner Andre Ware of Houston (46 touchdowns, 4,699 yards senior year at Houston - was a pro washout); and Colt Brennan of Hawaii - drafted by the Washington Redskins.