VPIP - Voluntarily put money into the pot. This shows you how often a player is in the pot, and therefore, whether the player is loose or tight in a general sense. For full ring a player above 20 would be considered loose, 18-14 fairly tight, and sub-13 tight.
PFR – Pre-Flop Raise Percentage. This is the percentage of the time a player will raise a hand preflop. For full ring play (again general guidelines here) a player with a VPIP above 18 is considered loose, 17-12 fairly tight, and 11-below very tight.
AF – Aggression Factor. This is a formula which shows how aggressive a player can be post-flop. Don't give this number absolute belief, it's more of a guideline stat. Someone with an AF of 5+ is very aggressive, 3-4 solidly aggressive, 2-3 somewhat aggressive, and 2-below fairly passive.