The Temple of the Jade Serpent is a new dungeon in the newest expansion Mists of Pandaria, located in the the Jade Forest. This is where the last emperor defeated the Sha of Doubt, but now the Alliance-Horde conflict has resulted in the Sha overtaking the Temple. The Temple itself is dedicated to one of the August Celestials, Yu'lon the Jade Serpent. It is the first dungeon Alliance/Horde players can do when they arrive in Pandaria.
As it looks like from the map Blizzard showed at BlizzCon 2011, it looks like when you arrive at the Temple, you can pick one of the two bosses from either side. Once the group kills one of the bosses, the door to the other will be locked and you have to do the instance again to kill the other one. This is only on normal mode; in heroic, the locked doors will be open and you will be able to kill the other boss.