Beside means ‘next to‘ something or ‘by‘ something it’s a meaning of beside. And beside is a preposition.
I sat beside my beautiful wife.
Who is that singer who was beside David on stage ?
Besides is similar to “as well as” and we can used to add new information to what is already known.
Besides David Bowie, Pink Floyd it’s also great English band.
Besides PHP I also know how to write programs in JS.