The net dipole moment defines the electronegativity difference between two atoms. Given a molecule with only two atoms, one need only about the electronegativity difference between the atoms. I've seen two thresholds: 1.6 and 2.0. If the electronegativity difference between two atoms in a binary molecule is 0, then the bond is nonpolar covalent: the electrons are equally shared. If the electronegativity difference between two atoms in a binary molecule is greater than 0 but less than 1.6 or 2.0 (I've seen both of these thresholds quoted), then the bond is polar covalent, and the atoms are shared unequally, favoring the more electronegative molecule. If the electronegativity difference between two atoms in a binary molecule is greater than 1.6 or 2.0 (depending on the threshold employed), then the bond is ionic, and the atoms are said to "gain or lose" electrons.