A molecule with atoms of varying electronegativity. Some atoms have a slightly negative charge and others have slightly positive charges. Water is a common example of a polar covalent molecule; the oxygen atom has a slight negative charge and the hydrogen atoms have slight positive charges
See Also
Non-Polar Covalent

The net charge of a covalently bonded molecule is always 0, but the charge in a Polar Covalent is lopsided so that one end of the molecule is positive and the other is negative. The atoms themselves do not gain or lose charge, their electrons just tend to drift around more on one side.

I believe that what is being referred to here are molecules with dipoles. This is where there is some separation of charge (a dipole moment) across the molecule. This does not just require that the atoms have different electronegativities (something atoms for different elements always have). It also requires that the polarisations of the electron cloud of the individual covalent bonds do not cancel out (due to symmetry).