What is Dissociative, associative and interchange mechanisms?

In a dissociative mechanism (D) the leaving ligand departs first, giving an intermediate of lower coordination number; in an associative mechanism (A) the entering ligand binds first, giving an intermediate of higher coordination number; in an interchange mechanism (I) the two ligands exchange in a single step, with a bond-breaking ($I_d$) or bond-making ($I_a$) character.