Answer: they are distinguished from one another by the number of fatty acids attached to the glycerol backbone.
A monoglyceride is a molecule formed by a glycerol backbone and one fatty acid covalently bonded to it through ester linkages.
A diglyceride is a molecule formed by a glycerol backbone and two fatty acid chains covalently bonded to it through ester linkages.
A tryglyceride is a molecule formed by a glycerol backbone and three fatty acid chains covalently bonded to it through ester linkages.