what is the difference between conjunctions and prepositions???? and what do u exactly mean by prepositions???

Conjunctions are words that are used to join two or more words, phrases, clauses or sentences. For example: and, but, either…or, neither…nor, as, because, after, before, since etc.

A preposition is a word that expresses relation to another word or element and is usually placed before a noun or a pronoun. For example: across, on, in, by, off, out, to, etc. 

Thus, while conjunction acts as a joining word, preposition adds meaning while establishing a relationship between different elements.