Each model shown is one of 4 stereoisomers of the general formula:
RR'R"C-CR'''R'R
with 2 stereocenters and 4 different atoms or groups of atoms bonded to each stereocenter.

Each stereocenter is bonded to at least one atom or group that is different from the atoms or groups bonded to the other stereocenter (R'' and R''' in the general formula above).

To understand the reason for the above stipulation, see below.

Each pair of models, top and bottom, represents enantiomers; all other pairwise combinations represent diastereomers.

The four models shown represent 3 stereoisomers of the general formula:
RR'R"C-CR"R'R
with 2 stereocenters and 4 different atoms or groups of atoms bonded to each stereocenter. Both stereocenters are bonded to the same other three atoms or groups of atoms

In this case, two of the mirror image models (the bottom pair) have an internal plane of symmetry and are thus achiral and represent the same molecule. Although this meso form is achiral, it contains 2 stereocenters. The top pair respresent enantiomers. Either enantiomer is diastereomeric to the meso molecule.