In a coordinate covalent bond, a pair of electrons from one atom is shared by two atoms. The distinction is one of accounting. It is not really meaningful in a physical sense, and at high levels in chemistry, the nomenclature is not much utilized. Reactions between Lewis bases and Lewis acids result in coordinate covalent bonds. When a bond is forming, if one of the atoms brought both electrons to the bond, then people will not disagree when you call it a coordinate covalent bond, especially within the context of coordination complexes. Complexes are formed when the Lewis acid is a metal. There is an especially large and important subgroup when the metal is a transition metal.