Poker isn't about cards: it's about players. How you act, and how you react to your opponents, is what determines if you win or lose.
In order to evaluate your opponents, you should think of them as existing on two spectrums: tight vs. loose and passive vs. aggressive. Tightness/Looseness determines how many hands they play, and Passiveness/Aggressiveness determines how often they bet.
The number of hands a player plays--and the strength of those hands--determine if a player is tight or loose.
A tight player plays fewer starting hands. They tend to be patient and wait for very strong hands, such as big pocket pairs or suited face cards. They might also play fewer hands from early positions, and prefer to wait until they are near or on the button.
A loose player is the opposite: they play more hands more frequently, even if the hands are not particularly strong. Some players might play just about every hand they're dealt! These players are also more likely to play hands when they are in an early position, such as under the gun.
Passive players don't bet as often: they tend to call or fold rather than raise. Sometimes passive players are passive because they are not as confident--other times, they are passive because they prefer to let others bet into them so they can win a big pot.
Aggressive players are the opposite: they love to bet and raise. Sometimes this is because they are confident and able players who know how to string along their opponents, built pots, and either win the hand or bluff. Other times, this is because they are just bullheaded amateurs!
Reading opponents is more of an art than a science, and it comes down to one word: practice, practice, practice. Sometimes an opponent is being aggressive because they have the nuts--other times they're steamrolling and ready to make a big mistake. Sometimes, a player is just calling you down because they're scared or confused--other times they're laying the perfect trap.
Always read opponents before you read your cards. Are they acting nervous? Are they acting tired? Or, have they just taken down a big pot, and seem to be acting a bit more sure than before? A general rule is that players represent the opposite of how they actually feel: so someone acting tough is probably weak, and someone acting bored or unsure is probably strong.
But avoid hard and fast rules, and always read your opponent as an individual. The biggest asset you can have is a long memory: how has that player acted in that position or situation before? You can then use this to guess how they will act again.
Of course, the very best players know how to change up their behavior: they can act passive and tight, and all of sudden go on a tear of big bets and big bluffs. This is smart poker--so not only should you know when your opponent is actively changing their play, but you should know when to change your own as well!
Table image is a player's reputation: if they're known as smart and savvy players, they'll get more respect; or, if they're known as wild players, they might get more calls. Smart players cultivate particular images, and you should too: know how your opponents see you, and play back at them accordingly!