Glucose is a Monosaccharide which contains four stereocenters. Stereocenters are chiral carbons to which all the four groups attached have different masses. Glucose is also known as Diastereomer because it contains more than one configurations about the chiral centers.
Also, when two compounds containing same type of configurations about all chiral centers except one are called as Epimers. For example, in D-Glucose when the configuration at carbon number 4 is changed then the resulting compound formed is the epimer of D-glucose and is called as D-Galactose. Or, if the configuration at carbon two is changed then the epimer formed is called as D-Mannose.
Below are the structures of D-Glucose and D-Galactose which are epimers of each other due to configuration change at carbon 4.
