This scheme I just drew up specifically for you should answer your question.
Diastereomers are stereoisomers that are not enantiomers of each other. That includes conformers (geometric isomers that derive from single bond rotation; usually interconverting rapidly) and atropisomers (under which I would subsume E/Z isomers; they derive from hindered rotation around a typically single bond and are separable) and anything with differeces in asymmetric carbons.
If you have exactly one asymmetric carbon with inverted stereochemistry, then the diastereomer is a special one: an epimer. Anomers (e.g. α- and β-D-glucose) should be considered special cases of epimers.
Let me use the menthol isomers (image taken from Wikipedia, where a full list of authors is available) to clarify the concept:
Two vertically aligned structures are always enantiomers. Two structures that are not vertically aligned are diastereomers.
(+)-Isomenthol is (+)-menthol’s C4-epimer, while ($-$)-neoisomenthol is (+)-menthol’s C1-epimer. And ($-$)-neomenthol is (+)-menthol’s C2-epimer. All of these are also diastereomers.
(+)-Neomenthol is not an epimer of (+)-menthol. It is still a diastereomer, however.