The carabao, or the Philippine water buffalo (Bubalus bubalis) is a large bovine traditionally used as a beast of burden. The kalabaw (in Tagalog) has become synonymous with rice farming, especially in the more remote rural communities.
The carabao stands from 4-5 feet at the shoulder, with black or bluish-black skin. It has two wicked-looking horns that sweep back in a crescent-shape from the head; when ridden, the reins are often attached to these horns. They can also inflict massive damage; wounded or feral carabao have often killed unwary handlers when they attack, swinging their heads from side to side.
The animal being slaughtered in the movie Apocalypse Now is a carabao.
The carabao is normally used to plow rice fields, as well as pull wagons or sleds. The sight of a plodding carabao, hauling rice from the farm to market is a common sight in most Philippine villages. They are slow (and tend to indulge in mud baths when it gets too hot) but they can endure long stretches of work before getting winded.
Although they don't produce much, carabao milk is a delicacy (at my grandmother's farm in Nueva Ecija, I remember we mixed it into the rice for breakfast) - it is much richer and creamier than cow's milk.
In recent years, carabao meat (carabeef) has also been touted as a cheaper alternative to standard cow beef - being tougher and stringier, it didn't really take off until the BSE hysteria started up. These days, it sells at about the same price as standard beef.
Some facts taken from a pamphlet published by the Philippine Carabao Center.