Diastereomers are stereoisomers that are not mirror images of each other.

eg. 1:

1 and 2 have the same molecular formula and the same structural formula and, therefore, are stereoisomers.  1 and 2 are not mirror images of each other.  Thus, they are diastereomers.

eg. 2:

3 and 4 have the same molecular formula and the same structural formula and, therefore, are stereoisomers.  3 and 4 are not mirror images of each other.  Thus, they are diastereomers.

eg. 3:

5 and 6 have the same molecular formula and the same structural formula and, therefore, stereoisomers.  5 and 6 are not mirror images of each other.  Thus, they are daiastereomers.

Cis-Trans isomers are a subset of diastereomers; all cis-trans isomers are diastereomers, but not all diastereomers are cis-trans isomers.

see also enantiomers