Cattle

are domesticated creatures, kept mainly for Milk and Beef. In order to obtain cattle, you must tame Aurochs. Cattle can be lead around by equipping a Rope, right-clicking on the animal, and selecting 'Leash'. Cows can have twins on rare occassions.

Cow
Cows, female cattle, are the only source of milk other than feeding Aurochs Four-Leaf Clovers. Heifers, cows that have not give birth yet, do not produce milk, thus you will need access to a Bull in order to get milk from a cow. Cows produce milk at a rate of 1L per half-hour (10 minutes in real time), and can 'store' a maximum of 10L at a time. Calves drink the milk, so be sure to leave enough for them.

A calf takes 10 real life days to grow up.

A non-pregnant cow eats 4.8 units of food per day (a food trough holds 200 units). A pregnant cow eats 10 times as much (48 units per day). A lactating cow eats 4.8 units of food per day, plus 1/10 unit for every 1L of milk produced.

Bull
Bulls, male cattle, are mainly used to pull Wagons, though

Cows can also do this. Bulls are also used to impregnate cows. It is a good idea to separate your bulls from your cows, to ensure that you can select the highest quality bull to mate with your cows when you want calves. You should also use your lowest quality bull to pull a wagon, since quality has no effect on wagon-pulling and if you have to flee and leave your wagon behind, then the loss will not be as bad.