I am confused about the differences (and similarities) between "wet-dry" and "sump" filtration. After reading threads here and looking at products at online stores, I am lost in the details. Particularly because Eheim seems to have a canister filter labelled as "wet-dry", but many others look to my eye like sumps, at least in general physical dimensions, etc.

When would one choose "wet-dry" over "sump" and vice-versa?