Ammonia is a polar molecule. The polarity is a direct result of the molecular geometry of ammonia. Many people think it is a planar non-polar molecule, but it is actually a pyramidal polar molecule.
"Polar" simply means there is a separation of positive and negative charge. Each nitrogen-hydrogen bond in ammonia is polar because nitrogen is more electronegative than hydrogen.
If the three nitrogen-hydrogen bonds were in the same plane, the polarities of each bond would cancel out and the molecule would be non-polar. However, there is a non-bonding pair of electrons on the nitrogen. These electrons push the three hydrogen atoms out of the plane, giving ammonia a trigonal pyramidal geometry and a polarity.