When two atoms with different electronegativities come together as a molecule, the difference in their electronegativity values becomes the dipole moment of the molecule (see drawing). Basically, one side of the molecule feels a partial negative charge while the other side feels a partial positive charge. (The symbols that represent this are also included on the figure with + and - signs). A molecule with a dipole moment is a polar covalent compound.
You can actually calculate numerically the difference in the electronegativity. See the numerical values for each element on the periodic table at the top. The difference in electronegativity determines the degree of polarity of the covalent bond. When the difference in electronegativity becomes very great, the bond is no longer covalent (or polar covalent) but is considered an ionic bond. In an ionic bond the electrons are no longer considered shared between atoms. In the ionic bond the electrons are donated from atom to atom.