Diastereomers (also called diastereoisomers) are stereoisomers which are not enantiomers. They are molecules which have the same atoms and bonds but different stereochemistry in at least one, but not all, of their chiral centres. Diastereomers can have very different properties even though they look similar. This is different from enantiomers, that have exactly the same properties except when in a chiral environment like the human body.