Examples: up, on, under, around, along, among, beside, between, over.
Prepositions connect a noun or pronoun to the rest of the sentence. All prepositions are part of a prepositional phrase. The prepositonal phrase contains nouns or pronouns.
Mary walked around the log.
Around is the preposition, and it is the first word in the prepositional phrase. Log is the object of the preposition. A noun or pronoun is always the object of the prepositon. The preposition, around, connects the noun, log, to the rest of the sentence. Without the prepositon the sentence would be - Mary walked around log.