In chemistry two stereochemical isomers are said to be enantiomers if they can be superimposed on the mirror image of the other. Stated differently: an enantiomer is a non-superimposable mirror image of itself. Two molecules that are made up of the exact same atoms and having exactly the same neighbors and differ only in their spatial orientation are said to be stereoisomers. A test for enantiomers as taken from http://www.chemistry.mcmaster.ca/berti/3f03/special_stereochemistry.pdf: Do the above molecules possess mirror planes of symmetry? That is, is it possible to find a plane that cuts through the molecule such that the two halves are mirror images of each other? It has to bisect all of the chiral centers.
An enantiomer of an optically active isomer rotates plane polarized light in an equal but opposite direction of the original isomer. A solution of equal parts of an optically active isomer and its enantiomer is known as a racemic solution and has a net rotation of plane polarized light of zero. A more indepth explanation of this is in the footnotes for Optical isomerism;
That footnote also carries indepth information on the proper prefixes for naming an optically active isomer. Enantiomers will have the opposite prefixes of each other. D-... becomes L-..., dextro becomes levo, (+)... becomes (-)..., R-... becomes S-....This notation is used to denote molecules with chiral centers. If the mirror image can be based around a carbon-carbon double bond, with two different ligands on each of the two carbon atoms making up the double bond then it is denoted as either a cis- or a trans- bond.
Research is expanding into the field of chiral chemistry quite rapidly because, for the most part, only one enantiomer is active in a biological system. This is because most biological reactions are enzymatic and the enzymes can only attach to one of the enantiomers. That is usually not a problem because mother nature only tends to make the one that you need. There are exceptions where both enantiomers are biologically active. One would be (+)-carvone & (-)-carvone; one smells like spearmint and the other like caraway.
The above comes from my Organic Chemistry class notes (exceptionally well taught by Dr. Richard Jagger aka Jag) and as such are copyright by me (Tres) and now licensed under the GNU FDL. Additional information may be found at:
Stereochemistry http://www.chemistry.mcmaster.ca/berti/3f03/special_stereochemistry.pdf http://www.chem.umd.edu/courses/chem233mazz/chapternotes/chapter9notes.pdf
