Which of several types of liquids you pour out has its own meaning. Wine represents the blood of the earth and is a suitable offering for almost any purpose. Whites as well as reds, by the way, have this meaning. Beers represent oath and community. They can be poured to strengthen friendship, to give or receive an oath, or to form a compact or agreement.
Milk is a symbol of family, kindness, and acceptance.
Such libations can be poured from any cup, bowl or dish, though in ancient times special vessels were often created for libations. These frequently had the shape of animals. In ancient Crete, for example, special devices were used, called rhytons. They were often conical or piriform in shape, though many of them were in the shape of an animal's head with the exit hole at the mouth and the inpour at the back of the head. A marvellous example of a bull's head rhyton has been preserved.