Beside is a preposition meaning ' at the side of', 'by', 'next to'.
What's that beside that window?
Besides can be used like as well as, when we add new information to what is already known.
Besides vegetable, we have meat and seafood.
Besides can also be used as a discourse marker meaning 'also', 'as well', 'in any case'. It is often used to add a stronger, more conclusive argument to what has gone before.(Usually used at the beginning of a clause)
I don't like those shoes; besides, they're too expensive.