rSB – Stolen Base Runs Saved (Catchers/Pitchers) measures two things: the pitcher’s contributions to controlling the running game, and gives the catcher credit for throwing out runners and preventing them from attempting steals in the first place.
rBU – Bunt Runs Saved (1B/3B) evaluates a fielder’s handling of bunted balls in play.
rGDP – Double Play Runs Saved (2B/SS) credits infielders for turning double plays as opposed to getting one out on the play.
rARM – Outfield Arms Runs Saved evaluates an outfielder’s throwing arm based on how often runner advance on base hits and are thrown out trying to take extra bases.
rHR – HR Saving Catch Runs Saved credits the outfielder 1.6 runs per robbed home run.
rPM – Plus Minus Runs Saved evaluates the fielder’s range and ability to convert a batted ball to an out.
DRS – Total Defensive Runs Saved indicates how many runs a player saved or hurt his team in the field compared to the average player at his position.
● Outfield Arm Runs (ARM) – The amount of runs above average an outfielder saves with their arm by preventing runners to advance.
● Double-Play Runs (DPR) – The amount of runs above average an infielder is in turning double-plays.
● Range Runs (RngR) – Is the player an Ozzie Smith or an Adam Dunn? Do they get to more balls than average or not?
● Error Runs (ErrR) – Does the player commit more or fewer errors compared with a league-average player at their position?
These links are all in the Glossary section under "defense".