We have already discussed the structure of chemical compounds, and how that relates to their reactivity.  However, there is one important aspect we ignored; stereochemistry.  Stereochemistry is the study of the orientation in space of the atoms of a molecule.  Drawing the structure of a molecule merely depicts the linkage between atoms, and doesn't necessarily define a species exactly.  It may be that there is no stereochemistry to worry about, but in many cases, there is.

If we represent the following imaginary molecule with no stereochemistry, there is nothing apparently special about it:

However, if we represent it fully, including the position of atoms in space, it becomes apparent that the description above is vague, as it encompasses two molecules; those below:

These two molecules are mirror-images of each other, but no matter how we attempt to orientate them in space, they will not super-impose.  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 one other important respect; 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.