Plus/minus (+/-) = The amount of cumulative goals over the course of a season a club scores with a particular player on the field. For example, should Manchester United score 50 goals over a season with Ryan Giggs on the field while conceding 34, his Plus/minus score would be +16.
Scored/90 (S90) = The amount of goals scored per 90 minutes a player is on the field. This varies slightly from goals scored per game, as often football players don't play entire games. Scored/90 allows us to observe the rate at which a team scores goals while a certain player is on the field.
Using the same example as above, were Ryan Giggs to play every minute of every game (ie. 3420 for the season), his Scored/90 and Goals-per-game rates would be the same at 1.315. However, if Giggs played only 3000 minutes for the year over those 38 matches, his Scored/90 rate rises to 1.5.
Conc/90 (C90) = As for Scored/90, only tracking the rate at which a team concedes while a certain player is on the field. Any of these numbers adjusted for rate allows us to compare players within teams - if Giggs' Conc/90 is 1.0 and Nani's stands at 1.1, we can suggest that United are better off defensively on the wing with Giggs on the left wing.
Goal Difference (GD) = As calculated by leagues for years, Goal Difference (sometimes here you'll see it denoted as Team GD) is the season-long difference between goals scored and goals conceded. Adjusted slightly, it becomes GD/game, or how many goals on average a club scores or concedes than their oppositon per game. You (really should) know how it works.
+/- per 90 (PMP) = Adjusts Plus/Minus for rate, allowing us to judge a player by his compatriots. Using Ryan Giggs again, with a Scored/90 rate of 1.315 and Conc/90 rate of 1.0, his +/- per 90 stands at 0.315.
Impact Rating (IR) = Assesses the difference between a team's performance while a player is on the field, with their overall performance.
Again, using the above example involving Ryan Giggs, if United score 16 goals more than their opponents for the season when he plays, but have a goal difference of +7 for the season, his Impact Rating is his PMP compared to his team's GD/game. For this example, if Giggs played 1500 minutes over 30 games, his IR would rate at 0.324 - or Man U would be 0.324 goals per 90 minutes better than their opponenets when Ryan Giggs was on the pitch.