Beside - This is a preposition which means 'at the side of' or 'next to'.
It can also be used in comparison with; on equal footing with:
Besides - This can function both as an adverb and as a preposition. As an adverb, 'besides' is defined as in addition to, as well; moreover, furthermore.
As a preposition, besides is defined as in addition to; apart from.
To avoid confusion - beside is used exclusively for the meaning at the side of. Similarly, besides, as a preposition, is used exclusively for the meanings in addition to and except for.
Doubts on this article