What is chirality?

Sometimes two molecules can exist as non-superimposable mirror-images. That is to say, the mirror image of a molecule cannot be exactly placed on top of the original molecule.

A good example of this is your hands: they are mirror images of one another, but cannot be exactly placed on top of each other. 

Pairs of molecules that exhibit this property are known as enantiomers. When a carbon atom is bonded to four distinct groups, this is known as a chiral centre, and two enantiomers of this arrangement are seen.