The alpha and beta glucose and the L and D glucose are isomeres, which means that they are different structurally but equal in its molecular formula.
The alpha glucose differs from the beta glucose in structure, because it presents an attached hydroxil group facing down, away from the main structure. But the beta glucose presents the hydroxil group above the ring, on the first carbon. They are used in similar process to achieve different results. For example, for plants to store sugar, they need chains og alpha glucose to build starch, and chains of beta glucose to build cellulose. Therefore, they are classificated differently because they will be involved in processes to achieve different structures.
The difference between the L glucose and the D glucose is that the three hydroxil groups and one hydrogen group that forms the structure are in different sides. The D-glucose presents the hydroxil and hydrogen groups on the right side, and the L-glucose presents it on the left side. As they have mirrored structures, each one will be used where it fits better structurally, but they have the same components. The letters D and L are used to simply differ the orientation of the hydroxil and hydrogen groups, making it easier to understand how this molecule will relate to others structurally.