Preposition is a word which shows the relation between a noun/pronoun and some other word of the sentence. Some common prepositions are on, under, beside, etc.
For example: The child is sitting on the bed. ‘On’ signifies the relation between the child and the bed.
Conjunction is a word used to join words, group of words and sentences. Some common conjunctons are – and, but, or,both,etc.
For example: Ram and Sita are married. Ram, Sita are connected by and.