Prepositions show the relationship between a noun or pronoun and another word in the sentence. They usually describe location, direction, time, or manner. Some examples of prepositions include: on, in, at, to, from, by, with, for, etc.
Verbs describe actions, states of being, or occurrences. "Is" is a form of the verb "to be" and indicates a state of being.
Here's an example:
* The book is on the table.
* "On" is a preposition showing the location of the book.
* "Is" is the verb indicating the state of being of the book.