NH3, commonly referred to as ammonia, has a polar covalent bond. This means that the valence electrons in the bond are shared between the elements but are attracted unequally between them.
The electronegativity of nitrogen (3.0) is higher than that of hydrogen (2.2). Therefore, when the two elements form a molecule by sharing electron pairs, the electrons tend to be attracted more strongly toward the nitrogen side than to the hydrogen side. The overall effect is an electric dipole in the molecule formed, with a slight positive charge in the hydrogen and a slight negative charge in the nitrogen. Hence, the molecule has a polar covalent bond.