Sterioisomers

 Sterioisomers are molecules that have the same molecular formula, but have a different spatial arrangement. This means that they are arranged differently in space, despite the fact the atoms are bonded in the same order. In order for this to occur, the molecule must contain a carbon that is bonded to four different groups, This is called a chiral centre, or an asymetric carbon. Sterioisomers can be divided into further groups according to the specific spatial arrangements. 

Enantimers are sterioisomers that are non-superimposable mirror images of eachother. They are usually described as having D or L configuration. Often one configuration exists more frequently in nature, for example the L form of most amino acids is more prominant in nature, possibly because they are slightly more soluble than the D form. You can differentiate between the two forms by investigating the direction in which they rotate polarised light. 

Molecules that have multiple asymmetric carbons are called diastereoisomers. These are not mirror images of eachother. A common example is a monosaccharide that has a carbon chain of three or more, such as glucose. The number of possible sterioisomers is equal to 2^n, where n is the number of asymmetric carbons in the molecule. There are two further types of diasterioisomers - epimers and anomers. Epimers are two diasterioisomers that differ at only one of the multiple asymmetric carbons, whereas anomers are cyclic molecules that differ at a new asymmetric carbon that is formed as a result of the ring formation. E-Z or cis-trans isomerism is another type of diasterioisomers, resulting from the restricted rotation around a double bond. 

References

Biochemistry, 7th Edition, Jeremy M. Berg, John L. Tymoczko, Lubert Stryer, W. H. Freeman and Company, New York, 2012, chapter 11- page 331, chapter 2 - page 27.