Beside is preposition indicating the location of something. Beside means, ‘next to’ or at the side of or close to something. Examples:
1. Your shoes are beside the closet
2. I feel lonely, please come and sit beside me.

Besides can be both preposition and adverb depending on the context used. Besides as preposition means, ‘in addition to’ or ‘apart from’. When used as an adverb, it means, ‘moreover’, ‘furthermore’, ‘another thing’, ‘as well’. Examples:
1. No one besides a good leader can take such a call for citizens welfare.
2. She is too tired to go for practice. Besides, she has hurt her knees.