I can't see any difference between the two, I would like someone who is advanced to shed some light ^^
In English, nouns can be used as attributes quite freely. In "zoology facts", "zoology" is a noun (substantive) qualifying "facts".
The translation of such a construction can be anything that generally relates "zoology" with "facts", for example:
- facts regarding zoology (as a science)
- facts from the point of view of zoology
- facts related in whatever way to zoology
- facts "belonging" to the general subject of zoology,
- et cetera
In the case of "zoological facts", "zoological" is an adjective. So, although the translation can be equivalent to the above, it can also be somewhat narrower: it can refers to facts that are zoological in nature.
A widely accepted zoology fact is that taxonomic classifications are complex.
(Here I am saying that classifying animal species is difficult, and that such difficulty is a well-known fact about the science of zoology in general).
A constant zoological fact is that carnivore animals eat other animals.
Here "animals eat other animals" is an observation that pertains to zoology, hence, it is "zoological". But it is not talking about zoology itself as a science and in general.
Sometimes, both forms acquire very distinct, idiomatic meanings, for example:
I am taking political economy classes.
I am taking classes on the subject of (political) economy.
I am taking economical classes.
I am being thrifty and taking classes that don't cost too much.
There is no general rule about when this happens, just the notion that adjectives (zoological, economical) tend to qualify more precisely and have specific meanings.