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Diastereomers are stereoisomers that are not mirror images of eachother.
If a molecule contains a single asymmetric carbon atom, it is will have two mirror image forms. If a molecule contains two asymmetric carbons, there are 4 possible configurations, and they can not all be mirror images of each other. The possibilities continue to multiply as there are more asymmetric centers in a molecule.
Tartaric acid contains two asymmetric centers, but two of the configurations are equivalent and together are called meso-, this configuration is not optically active, while the remaining two configurations are d- and l- mirror images. The meso form is a diastereomer of the other forms.
The families of 5 and 6 carbon sugars contain many diastereomers because of the large numbers of asymmetric centers in these molecules. See carbohydrates.
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