Creepers are hostile mobs. They will spawn in any dark area, such as underground or during nighttime. Unlike Zombies and Skeletons, they will not catch fire during daytime. They will drop 0-2 Gun Powder when killed.
An effective method for killing a creeper that is seen ahead of time is to create a wall that is at least three blocks high and a one block overhang on the creeper's side. This prevents the creeper from getting to the player by jumping over the wall, so the player may proceed to attack it. This method also works on other mobs.
If a creeper gets close to a player, they will make a hissing sound of a lit fuse. If the player doesn't back away far enough (or kill them first), they will explode, turning much of the environment around them into collectible items (but not all, some blocks will be utterly destroyed by this). If you set a creeper on fire,it will not explode(you may think it would).
From 1.2.6 onwards, the creeper skin has changed to a new Minecraft creeper skin.