After making an introduction to d-orbitals in the previous post, I guess it is time for some information about the types of interactions. As I said before, the symmetry (therefore the overlap) is essential for an interaction between ligand and metal orbitals.
There are three types of interactions between a ligand and metal orbital. These are Sigma, Pi and Delta interactions.
Sigma Interactions: These are the strongest interactions resulting from the best overlap between the orbitals. So the bonding molecular orbitals will be low in energy and the antibonding MO's will be high in energy. To give some examples, I can say two s-orbitals, two Pz orbitals, two dz^2 orbitals or an s and Pz orbital will give Sigma interactions. So, when we draw the bonding orbitals, they will be the lowest.
This image and the other one below are from Miessler and Tarr's Inorganic Chemistry textbook.
Pi Interactions: These are the second strongest interactions and we can usually see them between two Px, dxz, dyz orbitals. Since Sigma interactions are stronger than these, Pi interactions will strengthen Sigma bonds.
Delta Interactions: They are the weakest interactions and they usually occur between orbitals like dxy or dx^2-y^2. Don't let the image trick you. They are the weakest!