Runs Created estimates a player's offensive contribution in terms of total runs. It combines a player's ability to get on base with his ability to hit for extra bases. Then it divides those two by the player's total opportunities.
For example, in 631 "opportunities" (at-bats + walks), Player A had 377 total bases in 2014, with 66 walks and 195 hits. By this formula -- a very basic version of the formula, which doesn't include baserunning, double plays, etc. -- Player A created 156 runs in the '14 season.
At its most basic, with as few variables factored in as possible: TB x (H + BB) / (AB + BB)
Why it's useful
Invented by Bill James, Runs Created measures how well a hitter completes one of the central focuses of his job -- creating runs.