Brahmin serve a vital environmental niche in the Fallout world -- they form the foundation of survival for many species; most notably, humans. They also form the backbone of the New California Republic economy and of the New Reno "NCR brahmin rustling" economy.
Brahmin can pull carts, old cars, plows, and dead bodies. They can be driven into herds then used as stampedes on rival tribal villages. Brahmin droppings are great fertilizer and fuel for campfires - the fumes from their feces are also used to synthesize jet.
Brahmin are the mutated descendants of pre-War cattle. The cows that survived have given birth to mutated offspring. They've attained a second head, and abnormal growths. They also have eight stomach compartments and two hearts, twice as many as normal cows. Brahmin bulls have four testicles, and the udders of female brahmin have increased in size. They are of a toasted brown/orange color, and attack by head-butting or trying to gore someone with their horns.
Brahmin meat is delicious - if only when compared to other post-apocalyptic food. Brahmin are a source of milk similar to real-world milk, yet strangely different; it has even been said that brahmin milk may help cure radiation sickness.
Occasionally, a brahmin is born with a single head - essentially, a regular cow. Ironically, these single-headed brahmin are considered to be "mutated".
Brahmin are low level creatures and can be little more than a nuisance in the beginning of the game, if you can even find one that is hostile. In fact, most brahmin throughout the game are non-hostile and will only attack if provoked, as they are mainly found in ranches and small settlements. When they attack they run and ram into foes, swinging their horns into the body of the threat. Although their hide is thick, bullets will still penetrate and kill them rather quickly.