This may be a work of genius. The Universal Mob Grinder is a factory of death, an automated abattoir. This complex structure allows you to battery farm monsters for both experience and items. It can spawn, sort and slaughter them for you, or allow you to land the final blow and collect experience points for the kill, all with the minimum of effort.
All this is controlled by a series of switches and the whole structure is so simple to operate you might as well be working a vending machine. Here's what it looks like in action:
The zombie sorter is particularly impressive, sorting armoured zombies from unarmoured ones and taking villagers aside so you can purify them, while skeletons and creepers can be teleported to the Nether to override Minecraft's mob cap.
If you want to try playing around with the grinder yourself, or just seeing how this inventive structure works so well, you can find it here on Planet Minecraft.