Cows come in two different species: Bos indicus, from Asia (with a hump), and Bos taurus, from Europe.
Together, the two species include, at last count, 274 different breeds.
Nearly all North American cattle are from the European taurus type. For milk production, the most popular breeds are the Guernsey, Holstein, Ayrshire, Jersey, and Brown Swiss.
The breeds not only all look different but they also produce different milk.
For example, the Holstein has the classic-cow black blotches on white and gives the most milk per cow; a Guernsey has fawn coloring and distinctive yellowish milk; and a Jersey, also fawn-colored, has the highest percentage of butterfat in its milk.
For beef, the most popular breeds are Angus, Hereford, Charolais, Shorthorn, Santa Gertrudis, Brahman, and Brangus, a Brahman-Angus cross.
These breeds are known by varying physical characteristics. For example, the Angus is black and hornless, and the Hereford is recognized by its red body and white face.
The Brahman is usually all white, with distinctive floppy ears, while the Charolais is more of a cream color and has typical ears.
For exhaustive information about all breeds of major farm animals, check out the Oklahoma State University animal breeds site: http://www.ansi.okstate.edu/breeds/