Football Glossary

General

Expected Points (EP) assigns a value to each yard line to measure the points that are expected to be scored from that down, distance, and field position. Consider: there are thousands (millions?) of drives that have started 1st-and-10 from the 25, how many points did each drive end with? Some end in 7, some 6, many 3, most 0, and a few even -2. When all of those results are averaged together, you have an expected point value for 1st-and-10 at the 25 (it's generally around 0.6 points, by the way). 

Expected Points Added (EPA) the amount of Expected Points gained (or lost) on a play. 

Havoc measures the percentage of plays where the defense recorded a sack, tackle for loss, forced a fumble, intercepted a pass or broke up a pass. It can be divided between Front 7 Havoc (TFL/FF) and DB Havoc (INT/PBU). 

Predicted Points (PP) is collegefootballdata.com's specific implementation of Expected Points.

Predicted Points Added (PPA) same as EPA, using collegefootballdata.com's Predicted Points model.

Usage is simply the percentage of plays on which a player was a rusher, passer, receiver, or intended receiver (targeted). Usage can then be broken down lots of ways--by down, by play type, by looking at just successful plays, filtering out garbage time, etc. It gives you a great look at how much a player is being depended on and in what situation. 

Offensive Line Stats (via Football Outsiders):

Rushing: 

• Line Yards per Carry: the line gets credit for rushing yardage between 0-3 yards... and 50% credit for yards 4-8.... Anything over 8 yards is quantified as a highlight opportunity, and credit goes to the runner. As with the pro definition, lost yardage still counts for 125%. (Garbage time is filtered out for all line yardage averages.)
• Standard Downs Line Yards per Carry: The raw, unadjusted per-carry line yardage for a team on standard downs (first down, second-and-7 or fewer, third-and-4 or fewer, fourth-and-4 or fewer).
• Passing Downs Line Yards per Carry: The same unadjusted averages for rushing on passing downs.
• Opportunity Rate: The percentage of carries (when four yards are available) that gain at least four yards, i.e. the percentage of carries in which the line does its job, so to speak.
• Power Success Rate: [P]ercentage of runs on third or fourth down, two yards or less to go, that achieved a first down or touchdown.
• Stuff Rate: [P]ercentage of carries by running backs that are stopped at or before the line of scrimmage.

Passing:

• Sack Rate: Unadjusted sack rate for all non-garbage time pass attempts.
• Standard Downs Sack Rate: Unadjusted sack rate for standard downs pass attempts.
• Passing Downs Sack Rate: Unadjusted sack rate for passing downs pass attempts.