When a molecule has more than one chiral center, the possibility of having diastereomers exists. Diastereomers are non superimposable non-mirror images.
The total number of possible stereoisomers depends on the number of chiral centers (N);
# Stereoisomers = 2N
For example if you have a molecule with 1 chiral center like butan-2-ol, you would have two possible stereoisomers. They are related to one another as enantiomers. When N > 2 we have the possibility of having diastereomeric relationships.
Cyclohex-3-en-1,2-diol has 2 stereocenters, therefore the number of stereoisomers is 22=4. The (1R,2R)-cyclohexenol and (1R,2S) are related as diastereomers. Likewise (1R,2R) and (1S,2R) are also diastereomers. The (1R,2R) and (1S,2S) are related as enantiomers, as are the (1R,2S) and (1S,2R).
Unlike enantiomers, diastereomers can have different physical and chemical properties. You can separate diastereomers with standard chromatography techniques. You can separate them by crystallization, distillation etc. Recall that enantiomers on the otherhand only behave different in a chiral environment (i.e pocket of an enzyme, bound to a chiral catalyst, or on surface of chiral chromatography material).