What is π-donor and π-acceptor ligands?

A $\pi$-donor ligand has filled orbitals of $\pi$ symmetry with respect to the metal–ligand bond (lone pairs of $\ce{Cl-}$, $\ce{OH-}$); a $\pi$-acceptor ligand has empty ones of low energy (the $\pi^*$ orbitals of $\ce{CO}$ and $\ce{CN-}$). The transfer of electron density from filled metal d orbitals into the empty $\pi^*$ orbitals of a $\pi$-acceptor ligand is called back-donation.