Barrel

The barrel is closely tied to the crate, as both are popular containers to fill up rooms with when nothing else is at hand. Barrels (also known as cylindrical crates) usually come in three varieties: wooden, metal and exploding.

Wooden Barrels

Though not always made of wood, this type of barrel is made of a destructible material and often contains treasure or useful objects intended for the player.  The barrel acts as a sort of low-class treasure chest in this case and can usually be overlooked when the player just wants to get to the next level already.

Metal Barrels

The metal barrel, no matter which material it's actually made of, is indestructible.  In most cases, it is only present to make the room feel less empty or to serve as a platform proxy.  In rare cases, it can be pushed into a more strategic position for the puzzle-minded or rolled down a steep incline when physics and hapless enemies are present.

Exploding Barrels

Another type of barrel is the deadly kind.  Often liberally distributed throughout a level by some overzealous delivery service, the exploding barrel is an indirect weapon that some enemies will immediately recognize as cover until the player blows it up in their faces.  It has also seen use in multiplayer games where it becomes a strategic element, often appearing in highly defensible places to punish players who stick around for too long. Explosive barrels are almost exclusively red because fire is red.  However, these barrels, if not exploding with fire, are colored to represent the stereotype of their explosive contents.  An example of this would be a blue barrel containing a pulse of electricity, or a green barrel containing some kind of corrosive or radioactive acid.  These barrels, when shot, burn for a few seconds and then explode inflicting damage to all enemies or players within a constant, predetermined radius.  It has also been shown that contents of exploding barrels have no impact on essential, but not controlled, characters.
 

Barrels of disposable/harmful waste products

Barrels can also contain harmful or deadly ordinance inside them, which may or may not hurt the player if he/she gets too close. Typically the ordinance in question is nuclear waste or something of similar nature (often a green type of sludge that looks to be faintly glowing.)  
 

Barrels that are enemies themselves or contain enemies

Sometimes barrels in games will be in the guise of a typical metal or wooden barrel, but when the player draws near, the enemy will attack, try to bump into the player (to knock him off ledges) or whatever else it had in-mind. Barrels can also just contain enemies inside them and the enemies will be released if you destroy the barrel.