The "D" and "L" specifications in the names of D-glucose and L-glucose are used to differentiate between two different shapes of the glucose molecule. D-glucose and L-glucose are enantiomers, meaning that their molecular structures are mirror images of each other. The structural difference between these two molecules is best described in terms of the Fisher projection model, which is one way of drawing organic molecules.
In the glucose molecule, an oxygen and hydrogen atom group is bonded to a carbon atom. On the other end of the glucose molecule, there is a double-bonded oxygen atom. Looking at the Fisher model of D-glucose with the double-bonded oxygen atom pointed down, the oxygen and hydrogen group at the top of the atom points to the right. The Fisher model is the best option for describing the difference between D-glucose and L-glucose because it shows the structural difference most clearly compared to other structural drawing models.
D-glucose and L-glucose are made up of the same atoms. The only difference between the two structures is displayed through the Fisher model. Unlike D-glucose, the oxygen and hydrogen group of atoms in L-glucose points to the left in the Fisher model. If these two molecules faced each other, they would look like a reflection of one another. Although the D and L specifications are commonly used to describe different structures of sugars and amino acids, it is not always the most favorable configurational descriptor because it focuses on the configuration of only one carbon atom in the structure, when there could be many.