A -PO4 group on a larger molecule, where the P (phosphorus) is single bonded to each of the four O's (oxygen), and the other bond of one of the O's is attached to the rest of the molecule. This means that the entire group has a net negative charge of -3 (from the "free" second bonds on the oxygen atoms which are only bonded to the phosphorus atom). Often hydrogen atoms will be attached to the O's. Sometimes double bonds between the P and an O are present.