Ghosts look like a charged creeper, but without the creeper. They have low health and can be killed with one hit. They spawn in darkness, like all mobs. Also, they only are here for a single minute before they despawn.
When a ghost is killed, it drops an item that first appears to be glowstone. But it is, in fact, a strage mote that glows. When four mote is combined, it makes an item that is warm when you touch it, Congealed mass looks like a golden nugget.