Legacy:Cattle
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 occasions.
Cattle may also be killed by foxes, so if a fox spawns in your pastureland be ready to protect your herd. [Verify]
Cow
Cows, female cattle, are the only source of milk other than feeding Aurochs Four-Leaf Clovers. Heifers, cows that have not given birth yet, do not produce milk, thus you will need access to a Bull in order to get milk from a cow. Cows can only 'store' a maximum of 10L at a time. Calves drink the milk, so be sure to leave enough for them. A female cow will take 4.5 real life days to give birth.
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.
Milk
Cows produce Milk at a rate of Milk Quantity * .01 per 10 minutes. That is .1L/10 minutes at Quantity 10. .4L/10 minutes at Quantity 40, 2.0L/10 minutes at Quantity 200.
Cows can only store a maximum of 10L Milk at a time.
Bull
Bulls, male cattle, are mainly used to pull Wagons, though cows can also do this. Bulls are also used to impregnate cows, accompanied with a "moo" sound effect. 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.
Breeding
Breeding for animals with higher quality stats is much more difficult than it is to grow higher quality crops. The quality of any of the calf's stat is the average of the male and female's stat, +20 -> -5 (someone please check my numbers there), and then soft capped by the breeding quality of the male involved. Since you're typically looking for multiple stats, on top of getting more breeding stat, this can prove to be a slow task.