An example might clear things up.
If a carbon atom has four different groups attached to it, then there are two different ways in which these groups can be arranged around the carbon atom. Such a carbon atom is known as an asymmetric carbon atom. The two different forms of the molecule are known as enantiomers. They are mirror images of each other but cannot be superimposed on each other. The molecules below are both alanine (2-aminopropanoic acid), yet they possess unique bonding arrangements.

     NH2                    NH2
      |                      |
      |                      |
CH3---C---COOH         CH3---C---H
      |                      |
      |                      |
      H                      COOH