Beside or besides?

Beside is a preposition. It means ‘at the side of’ or ‘next to’. It is rather formal:

He would like to take a photograph of us. Would you come and sit beside me?

There was a small table beside the bed, on which there was a book.

Besides is a preposition or a linking adverb. It means ‘in addition to’ or ‘also’:

What other types of music do you like besides classical?

As a linking adverb, we usually put a comma before and after besides in writing:

I don’t think going for a walk is a good idea. It’s quite cold, and, besides, it’s getting late and we don’t want to be out in the dark.