http://www.stathead.com/bbeng/woolner/statglossary.htm

Statistics Glossary
by Keith Woolner

Pitching Statistics

ERA+ (and RA+) 
Park and League Normalized ERA (or RA, which can be substituted for ERA
throughout this explanation).  ERA+ = LgERA / ERA, where ERA has been
park-adjusted (and the decimal dropped).  This is the pitcher's counterpart
to PRO+, and measures how well the pitcher prevented runs from scoring
relative to the the rest of the league (and factoring in the home park he
plays in).  Note that while lower ERA numbers is better, the direction is
reversed for ERA+, meaning higher ERA+ represents better performance.   As
with PRO+, 100 is league average, and 120 is 20% better than average.

APR or PR/A 
Adjusted Pitching Runs.  APR = IP/9 * (LgERA - ERA), where ERA has been park
adjusted.  Measures the number of runs a pitcher prevents from scoring
compared to a league average pitcher in a neutral park over the same number
of innings.  The quantitative counterpart to the ERA+ rate stat.

WHIP (aka Ratio) 
Walks and Hits per Inning Pitched.  WHIP = (H+BB)/IP, sometimes stated per 9
innings --
WHIP = (H+BB)/IP*9.  Also known as BaseRunners/Inning (or BR/9).  Rose in
prominence with the popularity of fantasy leagues, particularly Rotisserie
leagues, where it is one of the pitcher's statistical categories.  This is
analgous to a pitchers-OBP (though a pitcher's OBP can be computed with given
complete data, WHIP is a more commonly seen substitute).

Game Score 
GS = 50 + 3*IP -2*(H+R+ER)-BB+SO  + (+2/each full Inning completed starting
with the 5th)
Bill James invented this pitcher's per-start metric as a response to the
adoption of Quality Starts, which James felt were to coarse a measurement for
evaluating the quality of a pitcher's outing.  Game Score is a useful rule of
thumb, since it is calculable from a box score, but it is not sabermetrically
precise (e.g. strikeouts are weighted twice as much as other outs).  It is
commonly thought of a measure for how "dominant" a pitcher was in the game
(sometimes colloquially known as "Ryanicity" since Nolan Ryan and his
multiple no-hitters were among the best Game Scores measured).  Kerry Wood's
20-strikeout,1 hit shutout in early 1998 was the highest Game Score ever
recorded for a 9 inning game coming in at 105.  A variant of Game Score
includes a -1 penalty for HBP and Balks, though this gets away from James's
original intent, which was to have GS calculable from the items in a typical
box score pitching line.

QS (Quality Start) 
A game started in which a pitcher lasts for six innings or more and allows
three runs or less.  Some later variants make it 6 innings with <=2 runs
allowed, or 7 or more innings with <=3 runs allowed.  A rough rule of thumb
for separating good starts from bad ones, with the noble intent of crediting
pitchers with how well they pitched and removing run support from
consideration (the major problem with W-L records).  QS are controversial
because the boundary case (6 IP, 3 ER) leads to an unimpressive ERA of 4.50,
which critics argue is not "quality".  While the point is well-taken, it is
still a useful dividing point because (a) most quality starts are not of the
6&3 variety, (b) 6&3 is an easy to remember and use midpoint and (c) it is
relatively close to what a league average performance (a natural dividing
line between good and bad) would be, particularly in the higher offense era
of the mid 90's.