The two structures shown (left and right) are stereoisomers of 2,3-dibromobutane. They are related as

The left structure is (2S,3R)-2,3-dibromobutane, the meso form; the right one is (2S,3S). Only one of the two stereocentres (C3) is inverted, so the two are not mirror images and not identical. Stereoisomers that are not mirror images of each other are diastereomers. The meso form is achiral while (2S,3S) is chiral, which also shows they cannot be enantiomers.