What is the difference between an enantiomer and a diastereomer ?

The key aspect of an enantiomer is that it is a non-superimposable mirror image.

One way to know if something is a diastereomer is to compare it to the enantiomer. Once you do so, you could usually say that:

A diastereomer is an enantiomer that has one or more (but not all) stereocenter(s) modified such that the stereoisomer is no longer a mirror image.

In a more general sense without comparison, you could say that:

A stereoisomer with multiple stereocenters that differs by the configuration of one or more (but not all) stereocenter(s) and is subsequently not a mirror image of the other is a diastereomer.

So, let's say you had these four stereoisomers:

You can probably tell that the top row has two enantiomers because they are non-superimposable upon a reflection over a vertical axis, yet they are mirror images.

You might also see that the top left and the bottom right stereoisomers are not mirror images. You can tell because their configurations don't match up, unlike what you would expect from a mirror image. That turns that molecule into a non-mirror image, and a horizontally-flipped diastereomer.

You could argue the same for the top right and bottom right molecules to say that they are also diastereomers. The only difference is that you are comparing to a nearly-identical molecule, rather than a horizontally-flipped one.

Finally, consider the bottom left and the upper left molecules. They have identical stereochemical configurations, and only differ in their relative stabilities! Can you see that it is just a rotation of the central #"C"-"C"# #sigma# bond?