In the U.S. that milk is synonymous with cow's milk, though in other parts of the world goat, buffalo, or sheep's milk is more common. Milk comes whole, lowfat, and nonfat or skim.
Unless otherwise specified, most recipes call for whole milk (which has 3-1/2 % fat), though 2% can often work just as well. Milk in the United States is commonly homogenized and fortified with vitamin D; it's also high in protein and low in fat, which means it will curdle if you try to boil it in a sauce. Though milk is often added to egg dishes and used in some baking, other milk products—including buttermilk, evaporated milk, and condensed milk—often play a greater role in the kitchen than milk itself.
For 1 cup milk, substitute 1/2 cup evaporated milk with 1/2 cup water or mix 1 cup water with 3 Tbs. powdered milk.
If a recipe doesn't call for a specific kind, whole milk is the best bet; it's the least likely to curdle.