Beside means: next to, alongside, adjacent to (preposition).
Besides means: in addition to, apart from (preposition) / anyway (adverb, used to give another reason or make an extra comment about something)
Let’s look at a few examples:
Besides our cat, we have two fish and a hamster. (In addition to our cat, we have two fish and a hamster)
The library is beside the town hall. (The library is next to the town hall)
Terry: “Tom, I really don’t want to go to the party tonight! Besides, Bryony will be there and you know I don’t like her!” (The second sentence is another reason for Terry not to go to the party)
To test yourself, I’ve prepared a little quiz for you: