Beside is a preposition of location – it means “next to” or “on the side of.”
▶️ There’s a printer beside the computer.
However, “beside” is a little bit formal. In casual everyday English, we’d usually say
that there’s a printer next to the computer.
Besides is an adverb that means “in addition to”:
▶️ Besides being expensive, that car is ugly.
Besides can also be a preposition that means “except for”:
▶️ Jenny has no friends besides her sister.
(= Jenny’s sister is her only friend)
▶️ Besides John, who’s a vegetarian, everyone else here eats meat.
(= John is the only exception to the group of people who eat meat)