Revised as of 1/4/2009
Quarterbacks are ranked according to
DYAR, or Defense-adjusted Yards Above Replacement. This gives the value of the quarterback's performance compared to replacement level, adjusted for situation and opponent and then translated into yardage. DYAR (and its cousin,
YAR, which isn't adjusted based on opponent) is
further explained here.
The other statistic given is
DVOA, or Defense-adjusted Value Over Average. This number represents value, per play, over an average quarterback in the same game situations. The more positive the DVOA rating, the better the player's performance. Negative DVOA represents below-average offense. DVOA (and its cousin,
VOA, which isn't adjusted based on opponent) is
further explained here.
The simple version: DYAR means a quarterback with more total value. DVOA means a quarterback with more value per play.