The main stats in defining someone's style are VPIP (Voluntarily Put In Pot, sometimes there's a $ sign somewhere) and PR (Pre-flop Raise). These are what you see in the HUD and the forums as xx/yy (zz). It means the player entered the pot xx% of the time (limp, call, or raise, not including blinds, I think) raised it yy% of the time (so yy < xx) over a sample size of zz hands (often zz will not be big enough to get a good feel, so you can see if it's like 80% or like 20%).
Basically, the average player on 2+2 is a TAG (Tight AGressive player) who plays only good hands and raises them up...around 20/15 are usual TAG stats. Often the bad players are looser and have high VPIP and low PR (50/5, for example), so they play a lot of hands and rarely raise them. When they do, they usually have the goods.
Other useful stats can be found by clicking on the HUD, which brings up a whole break down of stats. 2+2ers often cite the fold to cbet % on flop (how often you can take the pot down after a pre-flop raise with another bet on the flop) and the fold to cbet % on turn (same thing but on turn). If someone has low flop fold% and high flop fold%, for example, then you know what you need to do...
FYI, I use Pokertracker 3.