There are actually three different species with the common name buffalo. All three are large, dark brown or black horned mammals in the clade Bovidae, but they are not closely related.
Perhaps the most familiar buffalo is the American buffalo, or bison. Bison are native to North America, and are called buffalo because European settlers thought they looked like the two species native to the Old World. Bison are distinguished from their Old World counterparts by having a large shoulder hump and a shaggy, woolly coat.
The two Old World species that granted the American species a common name are the African buffalo and the Asian water buffalo. African buffalo, unlike their American cousins, have sparse hair. Males have a distinct horny covering called a boss that joins their horns in the middle of their heads.
Water buffalo also have sparse hair. Males are a third larger than females and both are frequently used as domestic farm animals, either for labor, meat or milk. The horns of this animal have the widest span of any bovid, measuring more than 6 feet wide. Unlike the African species, the bases of the horns of the water buffalo are near the sides of the head rather than the middle.