Creepers are green and white mobs that are loaded with TNT. Creepers will automatically turn around and follow you when you are a certain number of blocks away, and explode right in front of you if you annoy it enough. Creepers drop gunpowder when killed, and an interesting fact is that if a skeleton shoots a creeper, the creeper will drop a music disc! This is an easier way to get discs than looting chests in the generated structures (and switching to creative- yes I'm guilty of that)