Vet Bed is a type of bedding originally used at Vet Practices, but is now commercially available. It's a fleece type bedding, and is used so much because ALL liquids will soak through the bedding onto whatever's underneath. This leaves the animal dry and comfortable. In practice, animals will bleed, wee, drool, ooze, etc onto their bedding, and if it's not moisture-wicking like this is, they lay in a puddle of their secretions, and it can get uncomfortable and cold. Also accidents like drips coming out means that the animal's not cold and wet.
You can get Vet Beds at most pet shops, but the cheapest place I've found them is Ebay. There's a seller that sells them all the time who I've bought several from. It's always excellent service and the vet beds are good.