In a covalent bond when two atoms have a difference in electronegativity, then the shared electron is drawn closer towards the atom that has higher electronegativity. For example in HCl, the Cl atom is more electronegative than the H atom. So the shared pair of electron in the H-Cl bond is drawn closer towards the Cl atom causing it have a partial negative charge and the H atom has a partial positive charge.
Polar covalent, it is what happens when there is unequal bond between two atoms, where one has a slightly negative charge and one has a slightly positive charge.
