From Academic Kids
Asymmetric substances have two forms (at each point of asymmetry) which differ in their optical characteristics: the levorotatory form (the (-)-form) will rotate the plane of polarization of a beam of light to the left, while the dextrorotatory form (the (+)-form) will rotate the plane of polarization of a beam of light to the right. The two forms, which are mirror-images, are said to be enantiomorphic: each is the enantiomer of the other.
Many chemical processes take place without regard to a substance's optical characteristics, and result in a racemic mixture in which neither form predominates. However, pure forms of the enantiomers can be obtained: for example, by allowing crystalization, and hand-separating the left-handed and right-handed crystals. Biochemical reactions also prefer one form over another: most amino acids are asymmetric, and the L-form is used preferentially in biological systems.
Racemization is the process of a relatively pure substance becoming a mixture of enantiomeric forms; the rate of racemization (from L-forms to a mixture of L-forms and D-forms) has been used as a way of dating biological specimens.