Veal is meat from calves, the young of cattle. It is much softer, tender, pale and less strongly flavored than beef. This is because the calves are milk-fed and do not have much iron in their bodies. (Iron is used to make haemoglobin and myoglobin, respectively the red oxygen-carrying proteins in blood and muscle tissue.) They also have not developed much muscle.
You may find that some guests reject veal, believing that the calves are restricted to a small dark crate to cut costs or to keep the flesh pale and tender. (Of course, many people would not be bothered by this.) To varying degrees, such practices are illegal now.