This usually occurs in organic compounds, and occurs when an organic compund has a asymmetric or chiral carbon atom in it. A chiral carbon atom is one which has four different groups bonded to it, such as the carbon in chlorobromofluoromethane, which has a fluorine, chlorine, hydrogen and bromine bonded to it. These compounds form two isomers which are effectively mirror images of each other, but are non-superimposable (it sounds wierd but you really have to try it with a model to see what I mean).
The major distinguishing effect of optical isomers is that if you shine plane polarised light through a sample of just one isomer, the plane of polarisation will be rotated. In a 50/50 sample of both isomers the plane stays at the same angle as the isomers "cancel" each other out.
Optical isomers are especially important in biochemistry as many enzymes are specific to only one optical isomer of a compound.