Game texture

A game's texture is defined by the number of players in each pot and whether there's more checking/calling or betting/raising. It usually takes a while to determine the texture of a game. Watching while you wait is a good idea at a live game; at an online poker site you'll often find the number of players/flop and the average pot size displayed next to each game.

These are the factors that go into describing a game:

Loose/Tight
Loose: Many players are in each pot. Players seem to be playing too many hands and going too far with them.

Tight: Few players in each pot. Players are showing premium cards and the hands are often contested heads up.

Aggressive/Passive
Aggressive: There's a great deal of raising both preflop and on later streets. Large pots are the norm.

Passive: Calling and checking are much more common than raising. Pots tend to be smaller.

Put It Together
Most games can be described using one of the Loose/Tight pair and one of the Aggressive/Passive pair. There is also a type called "typical" which basically describes a game you have not classified yet.

Tight/Aggressive: These games feature frequent raises by players with high starting hand standards. They also feature few players in each hand. You will see little checking and more bets and raises. Pots often are won before showdown. Bluffs often work.

Tight/Passive: This type of game also features few players, but most players are limping. Checking and calling is more common than raising. Pots will usually be smaller than normal. Bluffs may be successful if the players are weak in addition to tight.

Loose/Aggressive: A game of this texture has lots of players in for multiple bets each. There are bets and raises on all streets. Players are playing marginal hands and pushing them as far as possible. The pots tend to be huge and the implied odds on most hands are very good. Bluffs will often be called "to keep you honest".

Loose/Passive: The players in these games will be playing marginal starting hands and will be trying to play them cheaply. They will rarely push edges and are more prone to check and call. Pots are often decently sized due to the sheer number of players in each hand. In these games it's important to show the best hand at the showdown. Bluffing is out of the question.