When players act in a way that gives away something about their hand, it is called a "tell." Much like a tilt, it is important not only to recognize your opponents' tells but also to realize if you have any.
Some players' hands shake when they are placing a bet if they have a very strong hand, while other players' hands shake when they are bluffing. Some players act strong when they are weak and act weak when they are strong. These players will often bet aggressively by practically throwing their chips into the pot or even toward you in hopes that their show of strength will make you fold.
Players who have a large pocket pair in Hold'em--like AA or KK--will sometimes closely watch each other place their bet into the pot as if making sure that nobody shorts the pot. These players have already won the hand in their mind and want to make sure that they collect every last bet. And then there are some players who hold their breath when they have a strong hand.
There is an unlimited number of possible tells, for it is human nature to react in some way when excited, and everyone is different. Picking up tells on your opponents can be done only by paying attention to detail. So practice concentrating on your opponents as much as possible while hiding any tells you may have. Learning to control your emotions under many situations takes practice, but it is very important because poker is a game of information, and you don't want to give any more of it away than you have to. If you are able to, your game will certainly improve.
But what about reading hands and situations? We will cover that in the next section.