The Clods filter is so darn simple that it does not even have a dialogue box. From the Background and Foreground colors it will make a clod shaped pattern that can be used as a starting point for some random surface texture. PS deserves a better clouds filter than this: there is no control over the scale, noisiness or any of the other things that a decent noise filter should have. The big shortcoming is not being able to edit its scale. In a document of very large pixel dimensions, it is impossible to make a clod filter effect that would be suitable for a large sky effect.
When it works, it can be used to pass off varied texture onto an overly even surface, as a base layer for a random texture or even (heavens!) as a clod! The image below on the left was made by passing a Sketch / Base Relief filter over a pale Clouds filter.
The Difference Clouds filter is just a Clouds filter that is run in difference blend mode. It needs to be run a few times before you get interesting results.