Stereochemistry, a subdiscipline of chemistry, involves the study of the relative spatial arrangement of atoms within molecules.
It includes methods for determining and describing these relationships; the effect on the physical or biological properties these relationships impart upon the molecules in question, and the manner in which these relationships influence the reactivity of the molecules in question.
Pasteur could rightly be described as the first stereochemist, having observed that salts of tartaric acid collected from wine production vessels could rotate plane polarized light, but that salts from other sources did not. This property, the only physical property in which the two types of tartrate salts differed, is due to optical isomerism.
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, allowing the relative position of these atoms in the molecule to be described unambiguously.
A Fischer projection is a simplified way to depict the stereochemistry around a stereo center.
On the left is a Fischer projection, on the right a more general 3-D "wedge" structural diagram. Both diagrams depict the same stereochemistry.
The image on the right is a "side view" of what is depicted by the Fischer projection on the left, as if one were looking at the Fischer projection from the left (the direction of the arrow). The bonds leading to atoms 1 and 4 point in a direction opposite from the bonds leading to atoms 2, 3, 5, and 6.
Horizontal lines in a Fischer projection always indicate bonds which point in the same direction, forward or out of the plane of the page. Vertical lines indicate bonds in the plane or pointing backward from the plane of the page.