Many proteins and other compounds are in isomer form either left handed or right handed. Most of the amino acids we use are left-handed (or "L") isomers. There are two forms or enantiomers of aspartic acid. The name "aspartic acid" can refer to either enantiomer or a mixture of two. Of these two forms, only one, "L-aspartic acid", is directly incorporated into proteins. The biological roles of its counterpart, "D-aspartic acid" are more limited. Where enzymatic synthesis will produce one or the other, most chemical syntheses will produce both forms, "DL-aspartic acid," known as a racemic mixture. To see the differences between these isomers, go here: http://en.m.wikipedia.org/wiki/Aspartic_acid
The D in D-Aspartic acid stands for dextromethorphan. D-Aspartic Acid (DAA) is a compound based on an amino acid that is known to have very positive effects on the natural testosterone levels of the body.
It means right-handed. Carbohydrates and amino acids are designated as D- or L- according to the stereochemistry of the highest numbered carbon in the Fischer projection. If the hydroxyl group (or amino group for amino acids) is pointing to the right in the Fischer Projection, it is designated as D. If the hydroxyl group (or amino group for amino acids) is pointing to the left, the sugar(or amino acid) is designated as L. Most naturally occurring carbohydrates such as D-Aspartic acid(http://www.guidechem.com/cas-178/1783-96-6.html) are of the D-configuration while most naturally occurring amino acids are L.