Diastereoisomers are inversional isomers that do not have a mirror image relationship. An example would be 3-chloro-2-butanol. I'm not sure how I can actually draw these here, but think of it this way: In one of the disasteromers, the OH and the Cl are coming out of the plane on the 2nd & 3rd C respectively and both on the same side; in the other the OH and the Cl are still coming out of the plane, but one (say the OH) is on one side while the other (the Cl) is on the other side \. Hence, they aren't mirror images of one another.
I hope this makes sense to you.