A covalent bond is one where the electrons responsible for the bond are shared between the two atoms in the bond. This is achieved when the orbitals of the two electrons are hybridised to form a bonding orbital that extends over both atoms.

Unless the two atoms are the same (and have the same atoms bonded beyond them) there is polarisation of the bond. Whether the resultant molecule is polarised (has a dipole) depends upon whether the polarisations of the individual bonds cancel or not.

The situation becomes more complex if the bond is not a single bond. In this case there has to be a further hybridisation of orbitals where the electrons are not directly between the atomic centres but are displaced to either side. The electrons are still associated with the two atoms unless that are other double bonds. It the multiple double bonds can form an aromatic system then the electrons for the aromatic bonds extend over all the atoms of the aromatic system.