Stereoisomerism

Stereoisomers are chemicals with the same atoms as each other but different shapes.  Stereoisomers have the same molecular formula as each other, with the atoms connected to each other in the same order, but with a different shape. Stereoisomerism is also called "spatial isomerism." Stereoisomers are different from each other because of the arrangements of atoms in space, not how they are connected.

 Enantiomers 
Enantiomers, are also called "optical isomers." Enantiomers are stereoisomers that are mirror images of each other but can't be turned around or moved so that they are the same, the way a right hand and a left hand of the same person are mirror images of each other. This is different from a shape such as a chair - two chairs can be mirror images of each other, but then you can turn them round and stack them exactly on each other. Compounds that are enantiomers to each other have the same physical properties, except for the direction in which they rotate polarized light and how they interact with different optical isomers of other compounds. Pure enantiomers have optical activity and can be separated only with the use of a chiral agent. An optically active compound has two forms: D-(+) form and L-(−) form.

 References 

Category:Stereochemistry