All of these are types of gerrymandering. They can be used to reduce the number of legislative seats that the opposition party is likely to win.
Pairing will reduce the number of opposition legislators because it puts two of them together in one district. This, of course, means that one of them must lose. Diffusion reduces the number of opposition supporters in districts, making the party in power more likely to win each district. Concentration puts supporters of the opposition together in one district. This makes sure that the opposition will win that district, but it tends to mean that other districts have fewer supporters of the opposition and are therefore easier for the party in power to win.