What is the difference between the D-form and the L-form of a compound?

Answer 1

The stereochemistry. Specifically, for carbohydrates, the configuration at the last (highest numbered) stereocenter determines whether it's D or L form. D means it has the same configuration as (+)-glyceraldehyde; L means it has the same configuration as (-)-glyceraldehyde. Note that D and L (capital letters) are not at all the same as d and l (lower letters). The capital letters relate the configuration to glyceraldehyde, the lower case letters specify whether the substance is dextrorotatory or levorotatory. For glyceraldehyde, D is d (+) and L is l (-), but this is not always the case for other molecules.