A poker tells is any habit, behavior, reaction or physical/audible expression that gives other players more information about your hand. Remember, in poker there are many contradictions and exceptions to the rule. For the most part these tells apply to average players, as more advanced players may attempt to give off false tells. These are in no particular order.
Leaning In - When a player leans in towards and over the table often putting their elbows/arms on the table to prop them self up higher than normal after making a bold move, this is a sign of bluffing or weakness. They are often puffing their chest out promoting the idea that they are strong and ready to go to battle over this pot. If you have a decent hand calls them.
Facial Expression - Again, many players try to disguise their entire face by wearing a cap and looking downward. This is to avoid the classic stare-down that poker pros are famous for. They may try to study your face for nervousness (detecting a weak hand), or even look for repetitive characteristics like a body "tic". You may have obvious unhappiness in your face when your hand is weak, and conversely, you may show a contrasting show of confidence when your hand is strong.
Weak is Strong / Strong is Weak - In general, this usually applies to novice players but frequently applies to pros as well. Basically, players like to be actors, and when they have a monster hand they tend to look disinterested. Alternatively, a player increasing the level of his voice while raising the pot, and trying to look intimidating.. may be running a bluff.
Anxiety - In poker, when someone has a big hand they are typically ready for confrontation and can exhibit some of these characteristics. You may see the chest expanding abnormally, or you may notice the players voice become slightly higher as he makes a comment. Some of the top players in the game will stare at the vein on the top-side of your face for blood pressure changes. During a bluff, the player may demonstrate anxiety, but if he knows he will fold the hand if re-raised (non-confrontational end) may look quite comfortable.
Trembling Hands - Also, a symptom of anxiety, beware of a player whose hands are shaking, this nervousness can represent a big hand. Their adrenaline is pumping so fast they can't control it.
Repetitive Betting Patterns - Usually the most revealing tells are based on the way a player habitually bets during particular situations. For example, maybe the player always checks when he has made the nuts, or a player may regularly fold after being re-raised.
Body Posture / Attitude - Some players show obvious changes in their posture based on their attitude/hand strength. For instance, you may notice their shoulders drop/slump when they are not confident (weak hand). Conversely, you may notice the player is very attentive and sitting in an erect position when he has strength. You may also notice that a player who bluffs a lot, leans forward in a confrontational way... remember Strong is Weak!
Chip Stacking - When you first sit down at a table, study the way the players stack their chips. Although it is a generalization, loose aggressive players typically maintain unorganized/sloppy stacks, while tight conservative players keep well organized/neat stacks.
Buying In - People that are meek about getting money out of their pockets or buy in for less than table max are often not confident in their game, or are weaker players. People that Buy in and show off how much money they have in their wallet are often looser and aggressive, they are a flamboyant personality type.