Leggings and tights are very similar in that they are skin-tight garments that cover the legs and sometimes the waist. Leggings are much thicker, footless, and can sometimes be worn like pants. Tights are somewhat sheer, cover the foot, and must be worn under another garment.
A big question comes in which is the distinguishing factor – transparency or feet. Usually transparent garments without feet are called "footless tights," though some manufacturers refer to them as leggings.
Neither leggings nor tights, however, should be confused with stockings, which are more like socks that go all the way up to the thigh, and unlike tights or leggings are not joint at the seam.