There are typically four different types of poker player:
Loose means that a player's requirements to put money into the pot are lower than average. For example, a loose player may like to play hands like K3s (suited) preflop in Holdem, and could be seeing anything from 30 to 100% of flops in that game type.
Tight is the opposite. A tight player plays fewer hands per hour than his opponents average. This can result in only seeing anywhere from 10% to 25% of flops in Holdem.
Passive players are those that rarely bet. They are happy to call and fold, but they are not often seen making raises of their own.
Aggressive players on the other hand, are those that raise and reraise significantly more than the average. They may be perfectly willing to reraise you whilst on a flush draw, or whilst holding second pair.
Obviously the degrees of aggressiveness or tightness vary from player to player, but the vast majority will fit quite neatly into these classifications. Coupled with issues like how weak or strong a player your opponent is, you will have a powerful tool to help you decide what your opponent is holding in each situation. Classifying other players is also an extremely important aspect when considering your strategy for a specific table, tournament or hand.
There are many different types of poker players. The two broadest categories are those of successful and unsuccessful players. A player who is successful does not just look at what is in his hand and on the board. Players who are unable to read other players or do not make it a point to watch even when they are not in a hand are often unsuccessful as Poker players. Under the category of successful players, a successful player is able to group someone into one of many different categories. The first category is:
Aggressive: Aggressive players will play almost any starting hand. The cards in their hand may be 2 7 of hearts and clubs and they will play it. They are the players who will contribute to some of your biggest wins and biggest losses because they are very hard to bluff out of a hand. If you are at a rock garden (table where everyone appears afraid of what others have) and one player is betting big every hand or every other hand and winning about every hand it is suggestible that if you have mid-pair you call their bluff at least once, to gain an accurate characterization of their betting patterns and because it is likely you have them beat. If someone is betting it up every hand and constantly winning and mucking it is suggestible to "look them up" if you think you might have it. These are the players that you don't want to try to bluff, because as bluffers themselves they will often turn the tables on you or look you up.
Passive Aggressive: Passive-Aggressive players are often the most experienced of all players, are able to get a good read on who is a rock and who is aggressive and will not throw their chips away. Passive Aggressive players often call the blind to the flop if their hand seems decent enough (middle cards or high) and will do their fair share of bluffing, but not as much as a player who is purely aggressive. These are the players who will go after you hard and smash into the aggressive players when they get a hand. Often times Passive-Aggressive players will be the big-stack at the table because they rarely call large bets into hands they don't have a good chance of winning and rarely make mistakes. The worst enemy of a Passive-Aggressive player is a "River Rat".
River Rat: A River-Rat is often considered the worst poker player at the table. A river rat will usually bet before the flop if they have good cards and upon seeing a flop they don't like they will call ANY bet you throw at them just hoping desperately for that miracle at the end of the river. It is a common misconception among weak poker players that you will have a tougher time making money off of these types of players, when in fact quite the contrary is true. Profit in poker comes when your opponents make mistakes, and River Rats are constantly calling bets when they do not have the odds in their favor.
Rocks: A rock is an extraordinarily tight, patient and reserved player at the table. When they do not get a premium hand pre-flop they won't even call the blinds. When you are involved in a hand with these characters you can almost tell immediately that they have either got two high cards, an ace and a high or middle card, or a pocket pair 8 or higher. These guys rarely play hands but when they do they will bet big. Their worst enemy is an aggressive player who might scare them out of a hand. While they will normally go far they won't go as far as most passive-aggressive players or very good aggressive players. River Rats give them a very large chunk of their change because they don't realize that when a rock is betting they have little chance of winning. The worst enemy of these players is themselves. They lose a lot of money to blinds for hands that they don't play.
Passive-Rocks: Passive Rocks are the guys who will bet very very rarely. These guys will get a premium hand and will call bets with it, with an almost certain chance to win the hand will only call and will not bet. Passive Rocks are the most dangerous when they are involved in a hand with an aggressive player or a player who misreads them. Thinking that they are "chasing" cards because they are calling rather than betting, someone may bet an all-in on them and they will call, putting a bad hurt on whoever it is that raises them all in. Passive-Rocks generally do worse than pure rocks.
Slow-Players: These are some of the craftiest players. Slow-Players have unlimited restraint on betting on a good hand in order to allow somebody to think they have the best hand. These guys know how to read others and know exactly how to look and bet like they have a bad hand or are chasing. Slow-Players are good at drawing as much money out of a hand as they can, especially when involved with aggressive players. These guys are the worst enemy of any aggressive player because of the toughness of getting a read on them. A slow-player may catch three of a kind aces on the flop and check, just to draw another player into betting so that they can either A.) Raise them, B.) continue their play, acting as if they have a weak hand and meekly calling or C.) if they have plenty of chips and want to mislead other players as to how they play they will fold this hand and show the table then saying something like "I KNOW you have the straight" simply to make other players think that they are rocks or easy to get out of a hand and therefore making it easier for them to bluff.
The Perfect Player This guy can read you by your actions, he knows exactly what kind of hands you play by observing you when he is and is not in hands with you and other players and he knows how you chat or how you bet, etc. when you have a hand, including whether you are notorious for slow-playing or not. This guy will not give any money to rocks because he knows if they are in a hand they almost certainly have an advantage over his hand and know when an aggressive player is bluffing and when he truly has a hand. He has elements of a slow-player, playing certain hands to draw money from someone who he knows will fold to a bet, has elements of a passive-aggressive player, betting big when its all river rats and aggressive players sitting around him, and a rock in the sense that he doesn't play bad hands in bad positions or with a "family table" meaning close to everyone at the table is in the hand. This guy has elements of every class in him. These are the players you should fear. It is hard to predict the betting pattern of these guys because it is modified depending on who they are in a hand with. The best way to learn how they play is to watch every single hand they are in and see how they act for each and every person, including yourself, because it will be modified when he is playing against you. You may even want to call some of his small bets just to learn what type of hand he will bet certain ways against you with so that he does not have a chance to muck his hand and leave you in the dark about him.