Nouns name things or categories and serve as subject of the verb or as the object of prepositions. Nouns can be replaced by pronouns.
Adjectives describe nouns.
There is no one definition of any preposition. Instead one has to ask oneself which preposition the target language needs in any given phrase or situation. She schrieb mir einen Brief, als sie in Frankreich war. Verbs are able to show tense, i.e. present, past, etc, and otherwise have the meaning of action, state of being, etc. Every full sentence has a tensed verb in it.