In addition to being a good read in general, Tom provides a good overview of what win probability added means, and what it does (and does not) count. From the article (sorry, it's behind a paywall):
At any given point in a game, each team has a win probability depending on the game state (quarter, clock, margin and possession). If a player makes a basket, he improves the team's win probability. If he misses the shot or turns the ball over, he decreases their odds of winning. Intuitive stuff, right? Michael Beuoy, who runs Inpredictable.com, has written an algorithm that adds up all the credit and debits that a player accumulates during a game to arrive at a summed total of win probability added (WPA). Beuoy then takes a step further and aggregates all of those game totals for the entire season.
To be clear, player WPA only looks at the basic stuff: field goal attempts (made or missed), free throws (made or missed) and turnovers. After that, it adjusts for the game state because the gravity of the situation matters. For example, if a player misses a potential game-tying free throw at the end of the game, it's a much more devastating blow than if he misses a free throw in the opening minute of the game. By pinning it to a game state, WPA offers context to the box score numbers.