Stereochemistry is the study of the orientation in space of the atoms of a molecule. There are molecules with the same molecular formula and structure but with different orientations of atoms in space. There are significant number cases, in which it happens.
It is found that there are molecules, in which we find two different molecules are mirror-images of each other, but they will not super-impose and remain as two separate chemical entities. They are known as enantiomers.
The physical and chemical properties of the enantiomers are the same in a symmetrical environment, but differ if there is asymmetry in the environment (e.g. a biological system). Enantiomers also differ in the direction that they rotate plane-polarized light. Both enantiomers will rotate the plane of the light to the same extent, but in opposite directions. Such molecules are said to be optically active.
Concepts in Stereochemistry
Enantiomers: "A species with a non-superimposable mirror image." An example of a pair of enantiomers is lactic acid.
Racemic mixture, or racemate: A racemic mixture, or racemate is a mixture of exactly equal quantities of both enantiomers, so that it does not rotate plane-polarized light in either direction (as the effects of both enantiomers cancel out.)