What is a Camelbak Bladder?

Camelback is the name of the brand, but bladders are simply malleable rubber containers that hold water for you. Typically (and most certainly with Camelback) the bladder has a tube connected to it so you can access the water via the tube through a backpack the bladder is contained in. They are good for camping, as they go back down to an easily-packed size when empty.