In ice hockey, how is plus/minus calculated when it comes to individual player's stats. Also, on a broader scale, what does plus/minus mean exactly? Unsure if I'm being redundant here.
You can calculate a player's plus/minus using the following rules.
If a player is on the ice for a(n):
Plus/Minus is not awarded for penalty shots and goals scored into an empty net do not change how plus/minus is calculated.
Player plus/minus is calculated each game and then added to the net plus/minus from all previous games in the season.
With a shortened season this year in the NHL, season total plus/minus will look very different than other (non-shortened) seasons since the statistic is additive over all games in the season. For example if a player averages a +1 rating over a season, a regular 82 game schedule will see that player have a +/- of +82. This year, with the shortened 48 game schedule, that same player will have a +/- rating of only +48 despite performing equally well in both seasons.
The NHL publishes who was on the ice for each goal for every game as well as that players game +/- so you can check how the stat is calculated. Here's an example from a recent game. Buffalo vs. Montreal - 02 Feb 2013
Plus/Minus is calculated by when a person is on the ice when a goal is scored. If you are on the ice when your team scores, you get a +1. If you are on the ice when your opponent scores, you get a -1.
However, there are exceptions: No changes are made for you if you are on a power play, or a penalty shot. If you are on the ice when your team scores a short-handed goal, you get a +1.
On the broad scale, teams try to use the metric to determine if you (or your line) is bad at defense, or good at offense. There are different types of positions in the game (defensive forward, for example), and if your plus/minus doesn't match up as well as your team thinks it should (or compared against similar players), your playing time could be affected.
While this question is hockey based, it should be noted that it's being adopted for basketball by more statistic-minded general managers and coaches.
Lastly, to get an idea of good/great values, here is a list of the records.