Stereochemistry

Stereochemistry is the study of how molecules are affected by the way their atoms are arranged in space. It is also known as 3D chemistry as the word "stereo" means "three dimensional". Using stereochemistry, chemists can work out the relationships between different molecules that are made up from the same atoms. They can also study the effect on the physical or biological properties these relationships give molecules. When these relationships influence the reactivity of the molecules it is called dynamic stereochemistry. 
In chemistry, some molecules have more than one isomer. This means that molecules can have different forms, even though all the forms are made of the same atoms. There are two kinds of isomers. Constitutional isomers have the same atoms, but they are joined differently. Stereoisomers have the same atoms, they are joined the same way, but the atoms are arranged differently in space. An important part of stereochemistry is the study of chiral molecules. These molecules look almost identical, except that one molecule is the mirror image of the other.
In most chemical bonds, the atoms of a molecule free to move around without breaking the bonds. When a molecule has a double bond or a ring structure, the molecule can be sorted into different isomers. These are molecules with the same chemical structure but different forms.
The study of stereochemical problems covers the entire range of organic, inorganic, biological, physical and supramolecular chemistries.
History.
Louis Pasteur was the first person to study stereochemistry. He observed in 1849 that salts of tartaric acid collected from wine-making equipment could rotate plane polarized light, but that salts from other sources did not. This property was the only difference between the two types of salt. It is due to optical isomerism. In 1874, Jacobus Henricus van 't Hoff and Joseph Le Bel discovered the difference was caused by the way that the atoms bonded to carbon in a tetrahedral (four faced) shape.
Uses of stereochemistry.
Stereochemistry was important in solving the thalidomide disaster in the 1960s. Thalidomide is a drug that was first produced in 1957 in Germany. Doctors used it to treat morning sickness in pregnant women. Later, the drug was shown to cause deformations in babies. One isomer of the drug was not dangerous, but the other caused serious genetic damage to the embryos. In the human body, thalidomide undergoes racemization: even if only one of the two stereoisomers enters a human body, the body changes some of it to other one. The thalidomide disaster caused governments to test drugs more carefully. Selected people take new drugs in an experiment (clinical trial) first before the drug is made available for public use. Thalidomide is now used as a therapy for leprosy. Women must use it with contraceptives to prevent pregnancy.
Describing a molecule's stereochemistry.
When an atom can have other atoms connect to it in more than one way, it is called a stereocenter. For example, if a carbon atom has four different groups attached to it, it becomes a stereocenter.
Cahn-Ingold-Prelog priority rules are part of a system for describing a molecule's stereochemistry. They rank the atoms around a stereocenter in a standard way. This allows the relative position of these atoms in the molecule to be described very clearly. A Fischer projection is a simplified way to show the stereochemistry around a stereocenter.