From the S&P+ website since there are a few questions on it:
http://www.footballoutsiders.com/stats/ncaa
The S&P+ Ratings are a college football ratings system derived from the play-by-play and drive data of all 800+ of a season's FBS college football games (and 140,000+ plays).
The components for S&P+ reflect opponent-adjusted components of four of what Bill Connelly has deemed the Five Factors of college football: efficiency, explosiveness, field position, and finishing drives. (A fifth factor, turnovers, is informed marginally by sack rates, the only quality-based statistic that has a consistent relationship with turnover margins.)
Here are the components of the ratings shared below:
Second-Order Wins (2ndO Wins): Defined here and discussed in further detail here and here, second-order wins compare the advanced statistical components of a given game, and the single-game win expectancy they create, to the actual results of the game.
This projected win total is a cousin of the Pythagorean record, a concept common in many sports. They are presented below, with the difference between a team's wins and second-order wins in parentheses.
S&P+ rating: Using the five-factors concept above, the S&P+ ratings take into account efficiency (Success Rates), explosiveness (IsoPPP), and factors related to field position and finishing drives. It is now presented in two forms: the first is a percentile, and the second is an adjusted scoring margin specific for this specific season's scoring curve.
Off. S&P+ rating: A team's offense-specific S&P+ rating, presented in the form of an adjusted scoring average.
Def. S&P+ rating: A team's defense-specific S&P+ rating, presented in the form of an adjusted scoring average (and since this is defense, the lower the average, the better).
Special Teams S&P+ rating: This is an initial attempt to measure play-for-play special teams efficiency, weighted for overall importance.