• Offensive Efficiency (Team): Points Scored per 100 Possessions
• Defensive Efficiency (Team): Points Allowed per 100 Possessions
• Tempo (or Possessions): Estimated Possessions Per Game. Calculated: FGs - Off Rebs + Turnovers + (.4 * FTAs)
• eFG%: Takes into account the "extra worth" of the three point shot, by giving three point shots 50% more credit.
• TO%: Percentage of possessions the team turns the ball over. Calculated: Turnovers/Tempo * 100
• Off Reb %: Percentage of rebounds attained from a team's own misses. Calculated: Off Rebs/(Opponents Def Rebs + Off Rebs)
• FT Rate: Rate at which a player or team gets to the free throw line. Calculated: 100 * FTA/FGA
• Block %: Percentage of possessions a team records a block. Calculated: Blocks/Tempo * 100
• Steal %: Percentage of possessions a team records a steal. Calculated: Steals/Tempo * 100
• 3FGA/FGA: Percentage of field goal attempts that are behind the arc. Calculated 3FGA/FGA * 100
• A/FGM: Percentage of field goals from assists. Calculated: A/FGM * 100
• A/TO: Assists per Turnover
• TS%: True Shooting Percentage calculates what a player’s shooting percentage would be if we accounted for free throws and 3-pointers. True Shooting Percentage = (Total points x 50) divided by [(FGA + (FTA x 0.44)]
• P/40: Points per 40 minutes
• A/40: Assists per 40 minutes
• R/40: Rebounds per 40 minutes
• A Ratio: Assist Ratio is the percentage of a player’s possessions that ends in an assist. Assist Ratio = (Assists x 100) divided by [(FGA + (FTA x 0.44) + Assists + Turnovers]
• T Ratio: Turnover Ratio is the percentage of a player’s possessions that end in a turnover. Turnover Ratio = (Turnover x 100) divided by [(FGA + (FTA x 0.44) + Assists + Turnovers]
• GmSc: Game Score, the formula is PTS + 0.4 * FG - 0.7 * FGA - 0.4*(FTA - FT) + 0.7 * ORB + 0.3 * DRB + STL + 0.7 * AST + 0.7 * BLK - 0.4 * PF - TOV. Game Score was created by John Hollinger to give a rough measure of a player's productivity for a single game. The scale is similar to that of points scored, i.e., 40 is an outstanding performance, 10 is an average performance, etc.
• Usg %: Usage Rate is the number of possessions a player uses per 40 minutes. Usage Rate = {[FGA + (FT Att. x 0.44) + (Ast x 0.33) + TO] x 40 x League Pace} divided by (Minutes x Team Pace)
• +/-: The difference in score while a player is on the court.
• +/-/GM: Per game difference in score while a player is on the court.
• +/-/40: Per 40 minute difference in score while a player is on the court.
• PPS: Points Per Shot. Calculated: Points/FGA
• % Safe: Bill James created a formula to determine when a game is safe, to calculate...
  • Take the number of points one team is ahead.
  • Subtract three.
  • Add a half-point if the team that is ahead has the ball, and subtract a half-point if the other team has the ball. (Numbers less than zero become zero.)
  • Square that.
  • If the result is greater than the number of seconds left in the game, the lead is safe.
• ORTG (Player): Points Produced per Player Possessions. This formula is based on Dean Oliver's work from his book Basketball on Paper, which is recommended to all that enjoy tempo-based statistics. The formula takes up about 2 full pages in an appendix of his book, so it will not be duplicated here. Needless to say with the work that Mr. Oliver has done, he deserves the income, so go buy his book.
• DRTG (Player): Points Allowed per Defensive Possesion.
• Stop %: Individual defensive stops per defensive possession.

A special mention should be given to Ken Pomeroy's wonderful site as well, as it is a wonderful resource to all basketball fans. Also, his glossary of terms can be found here.