In terms of structure, the difference between D-glucose and L-glucose refers to the position of the OH-anion on the last asymmetric carbon of the glucose molecule. In D-glucose, the OH (hydroxyl group) is situated to the right of the carbon atom while in L-glucose, the OH is situated to the left.
The designations D and L extend to families of sugars. They stem from two different forms of an asymmetric carbon found in glyceraldehyde, a three-carbon form of sugar from which glucose and many other sugars are derived. In D-glyceraldehyde, the hydroxyl group of atoms is situated to the right, while in L-glucose, the hydroxyl group is oriented to the left. All the sugars derived from D-glyceraldehyde are D-sugars, including D-glucose and likewise L-glucose is derived from L-glyceraldehyde.
The significance of this difference lies in their shape. Unlike the two-dimensional Fischer projections which simply look like flat mirror images, in reality the substances actually have a different shape, one of the means that enzymes use to recognize substrates. Consequently, enzymes - and by extension biological systems - will only recognize and react to the D-form of sugars in virtually every instance. That is why L-glucose seldom enters into the discussion of biology.