Beside means “by the side of” something or someone. It is always a preposition.
Besides as a preposition means “in addition to.” As an adverb, it means “moreover” or “in addition.”
My grandmother would often sit beside me and tell me stories of the “old country.”
Besides playing sports with the children, I would also help them with their homework after school.
I decided to stay in the class. Besides, I had begun to enjoy chemistry more than I had imagined possible.