Prepositions are function words which are put before nouns or pronouns. These preposition + (pro)noun forms work as adjectives or adverbs like "with a long tail" in the sentence "I have a cat with a long tail." or "in China" in the sentence "I live in China."

Prepositions include: of, in, to, for, about, with, by, on, from, at... that's just a starter list. :)