That is a difficult question to answer and probably an even harder one to teach. The simple answer is, it's how the other players at YOUR table are playing and how you can best take advantage of their weaknesses and avoid their strengths. The problem is finding out what their weaknesses and strengths are. And herein lies the question - how do you figure out what a player’s weaknesses and strengths are? The answer, while simple, is not nearly as simple to apply and to learn. It's accomplished by watching, observing; paying attention to the players at your table.
There are seven main things you should be studying when you sit down at any table.
#1 - Hand selection - When their cards are shown, what hands are they playing? What position were they in when they played these particular hands? If the cards are not shown, the frequency with which they play hands in early position, middle position, late position, and out of the blinds.
This helps you determine the classification of the player - loose, solid or rock. Doing so will help you best utilize this information when you are playing a hand. To help you determine whether a player is in a particular category, ask yourself the following questions after having watched them play a couple of orbits: 1) How many hands have they played 2) What hands did they show 3) From what position did they play the hands.
With the answers to those three questions you should be able to place each player in an appropriate category. So given this general knowledge, what do you play against them? A lot of that depends on the answers to other questions (betting patterns, calling tendencies, etc;), but it’s important that you know their general tendencies (loose, solid or rock) when evaluating the other factors.
#2 - Aggression - You should be noting the frequency with which each of your opponents raise and/or re-raise. How often is a player raising and re-raising? The more often they are, the less credence you should give their aggression. The less often they are, the more credit you should give them. You should adjust your play (raising and/or calling) and the hands you play accordingly. Against an overaggressive player, you can be more liberal in the hands you re-raise or call raises with. Against a person who seldom raises, you need to be much more selective with the situations you raise and re-raise with.
#3 - Calling tendencies - You should be noting the frequency with which each of your opponents calls bets, as well as the amount of the bets they are calling. Additionally, you should note, when possible, what types of hands they are calling with and the amount that will usually induce them to fold.
Is your opponent a frequent caller or folder? If they are a folder, you should be more concerned when they make a call. If they are a frequent caller, you should always factor that in when deciding when and how much to bet. The old adage never bluff a calling station is a very true one. But you can value bet them with weaker hands than you could a tight player. Find their breaking point. Bet the amount you want them to call when you want a call. Bet the amount you know they will fold to when you want a fold.
#4 - Reaction to opponents bets/raises - How does a player react when they are raised or re-raised? How does a player react when they were the pre-flop aggressor and their opponent leads out into them or check raises them? Basically, what is their typical reaction to aggression against them? Do they fold routinely to bets/raises if they aren't strong? Take advantage of that and bet/raise them liberally. Do they constantly come over the top of bets/raises? Trap them.
#5 - Pre-flop betting patterns - How much do they bet? Does it vary based upon position? Does it vary based upon the strength of their hand? How much do they re-raise? When do they re-raise?
Determine if there is a variance. If there is, determine when they bet a certain amount, why they are. This will give you crucial information you can use against them. The most common things you will see pre-flop is over bets when they have a hand that is good but prone to being drawn out on (i.e. J-J/10-10/big ace - you should be more likely to call these bets in position because you will be able to take the pot from them post-flop) or over bets in early position from strong hands that, while they don't mind action, they want to reduce the number of potential opponents they have post-flop (mainly with hands like K-K/Q-Q, with certain speculative hands like small pairs and suited connectors, you would want to occasionally call these raises when the money is deep due to the implied value).
#6 - Post-flop betting patterns - How much do they bet? Does it vary, if so, are you able to determine when? When do they raise? How do they play weak hands? Strong hands? Draws?
Again determine if there is a variance and figure out why there is. The most common post-flop betting patterns you'll see are minimum bets when they have a small piece of the flop or a draw and want to prevent an opponent from betting them out of the hand (raise liberally - if they come back over the top of you, it usually means they were inducing the raise) or an overbet when they are trying to protect a vulnerable hand like bottom two pair or top pair/weak kicker (you can trap these people with hands like sets and straights because they typically won't be able to fold their hand due to the size of the pot).
#7 - Blind defense - How often does a person just call a raise out of their blind? How often do they reraise a positional raise of their blind? How often do they fold their blind when it is raised from late position? Other positions? Is there a certain amount they will call no matter what? Is there a certain amount they will fold to unless they have a premium hand?
If they fold their blinds to most raises, you should be attempting to steal their blinds as much as you possibly can. If they call but do not re-raise raises of their blinds, you should still raise more liberally due to the positional advantage you have. If the person likes to over-defend their blinds, you should be selective in when you do so, but be more aggressive (much more likely to get action) when you do decide to.
So there you have it. Everything you need to be looking for and what to do in a nutshell. Simple right? Ok, maybe not... but the more you look for these things, the easier it will be for you to figure them out and use them to your best advantage. It's a lot of work but the dividends you'll receive will make it worth it in the end.
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