Stereoisomers are molecules which have the same molecular formula, with a different arrangement of atoms in space. There are two types of stereoisomerism, geometric and optical.
Geometric isomers occur when a molecule contains a carbon-carbon double bond, and the rotation of the atoms around this double bond are restricted. This means the molecule can be present in two forms, commonly referred to as E and Z isomers. When the higher priorty sub groups of each carbon are located on opposite sides of the double bond it is known as an E isomer. On the contrary, when the higher priority sub groups of each carbon are located on the same side of the double bond they are siad to be in the Z isomer form. E/Z isomerism is also known as Cis - Trans isomerism where E equates to trans ans Z means cis.
Optical isomers occur due to the molecule containing a chiral carbon. This is when a carbon atom within the molecule has 4 different groups bonded to it. The two isomers (or enantiomers) are mirror images of one another, and so cannot be superimposed onto one another. Optical isomers can be distinguished due to their effect on the rotation of plane-polarised light, as one isomer rotates clockwise under plane polarised light whilst the other rotates in the anti clockwise direction.
Stereoisomers are important in the body as amino acids can exist as either D or L optical isomers (L is the naturally occuring form). This occurs as all amino acids (apart from glycine) contain 4 different groups bonded to a central chiral carbon. Also, due to the high specificity of enzymes, only one isomer is often the target of an enzyme within a specific reaction.