Optical isomerism is a paritcular type of steroeisomerism (stereoisomers are molecule that are made up of atoms joined up in the same order so have the same formula but are arranged differently in space because of how the atoms interact with one another.)

Optical isomers are specific because they have different effects on plane-polarised light (light which is emitted in a single direction using a polariser instead of every direction as how we see it.) 

For optical isomers to occur there must be 2 conditions met:

1) The molecules must contain a carbon atom.

2) This carbon atom is bound to 4 different atoms/groups of atoms- this is called a chiral carbon atom. 

When this happens you get optical isomers. Chemically, they behave the same (ie they take part in the same reactions) but one isomer rotates the plane-polarised light clockwise and the other anti-clockwise.