Two numbers are enough to get an idea of how someone plays: VPIP (how many hands they play) and PFR (how many they raise before the flop). The gap between them tells you whether they get into pots by raising or by calling.
The style map
These are the cut-offs Flopmortem uses to classify your own style at 6-max: if you call more than 8 points above what you raise, you're passive preflop no matter how many hands you play; otherwise, your VPIP decides.
The styles, one by one
Nit (VPIP below 18). Plays very few hands, almost only strong ones. Easy to read: when they raise or call a big bet, they usually have it. Steal their blinds and fold when they show strength.
TAG (tight-aggressive, VPIP from 18 to 27 with a PFR close to it). The healthy style in cash games: few hands, played aggressively. It's the profile worth copying; against it, avoid big pots without a good hand.
LAG (loose-aggressive, VPIP from 27 to 30). Plays more hands and puts on lots of pressure. Profitable for a good postflop player; expensive otherwise. Against it, call more with medium hands and let it overdo the bluffing.
Maniac (VPIP above 30 without calling too much). Raises with almost anything. Lots of variance, but lots of money too: wait for good hands and let them bet.
Passive preflop (calls much more than they raise). Gets into lots of pots by calling and rarely raises. Gives up the initiative: your c-bets work well against them.
Calling station (passive with a high VPIP). Calls with almost anything and almost never lets go. Never bluff them, and bet big for value: they're the opponent who gives away the most money at low stakes.
Your own style first
Classifying your opponents helps, but the biggest room for improvement is in you. If your VPIP and PFR are far apart, or you play like a nit on the button, that's your first leak to fix. Flopmortem calculates your style from your own hands and tells you what to adjust.