pokertracker?

VPIP/PFR/Total AF

Voluntarily (spelling?) Put Money in Pot % (basically the percentage of hands the player plays)
Pre-Flop Raise % (the percentage of the time the player raises the hands he plays pre-flop)
Total Aggression Factor (total aggression factor of flop af + turn af + river af, and the AF is found by a player's (bets + raises) / calls

Pokertracker gives you many stats to describe how a person plays, but in general people consider three of them to be the most important for getting a good rough idea of how a player plays.

VPIP - voluntary put into pot: what percent of hands the player puts money in (other than posting blinds)
PFR - preflop raise: what percent of hands they raise preflop
AF - aggression factor: roughly a gauge of how aggressive a player is. Aggressive actions are betting and raising. Calling is a passive action. I'm pretty sure checking and folding do not matter to AF. The lower the AF, the more a person calls and plays passively.

the standard format for reporting someone's stats are VPIP/PFR/AF and sometimes a fourth number which is the number of hands you have on the player (few hands and the stats are not trustworthy).