Isomerism exists when two or more compound substances (usually it is the case of organic substances) have the same gross formula (equal number of atoms of the same type) but different structural formula.
The most common forms of isomerism are: chain, tautomer, position (cis and trans), and optical. Substances from the substitution reaction of aromatic hydrocarbons (with benzene-core), present isomerism of position of another kind (ortho, meta, and para). Those with naphthenic nucleus have different isomerism of position (alpha, beta, gamma, etc). What I have just said is a recipe for stomachache. If you are still OK then let's go ahead to the optical isomerism.
Those organic substances which rotate the polarized plane of light to left (levogyre) or right (dextrogyre) present optical isomerism. So, the same gross formula, the same structure can have the same position isomerism but it differs only by optical isomerism.
Substances with optical isomerism are obtained in laboratory, are racemic mixtures (equality between levogyre and dextrogyre). There are bacteria that feed only with levogyre and others only with dextrogyre. This fact was used by scientists to separate racemic mixtures.