"Impression and pseudomorphism" in fossils
Impression are just that, an impression, an imprint of the fossil but the fossil that created it is gone (or just separate) such as this impression of a ammonite.
Pseudomorphism is just a new term for a natural casting it can be a cast of the fossil or a cast of the fossils impression, again the shape is preserved the the original fossil is seperate or has dissolved and been replaced, for instance the natural casting of the braincase of hominids can be used to reconstruct missing parts of the skull. This snail shell is also a pseudomorph, the entire thing has been replaced with agate.
In true fossilization some component of the original material is still present this can preserve internal structure. With bone for instance a ture fossil of a bone often has some of the original calcium/phosphate with pore spaces around it filled in with other minerals.  Of course like anything in nature pseudomorph and true fossil are not completely discrete but ends on a spectrum, partial replacement is the norm.