What are chiral isomers? How do I recognize Chiral centers?

A molecule and its non-superimposable mirror image is called enantiomer.  A simple example to illustrate enantiomer is left and right hand are mirror images of each other and cannot be superimposed.

The easiest way to recognize is by identifying asymmetric carbon with 4 different substituents attached to it. There may be multiple asymmetric carbons in a single structure which would lead to a number of enantiomers. If there are no asymmetric carbon, then the plane of symmetry should be considered to figure out the chiral center. In general a molecule with plane of symmetry is achiral (no chiral center). Aside from carbon, nitrogen and sulfur compounds can also confer chirality.