Since it's release back in the summer. Minecraft has been getting updates to the game. Cosmeticaly and updates to the gameplay. This is a short introduction to the skins.
At the time of writing there are three skin packs available for purchase on the XBLA. Each pack costs the same and they all contain different costumes either from the creators or from other games. Some game characters involve the "Left 4 Dead" cast of characters, Gordon Freeman, and Splosion Man. Other costumes involve zombies, squids, pigmen, and crusader knights.
The skins are mainly used in multiplayer modes to show off to other players. But one must remember that the skins do nothing to enhance gameplay.