beside and besides

Beside is a preposition meaning ‘at the side of’, ‘by’, ‘next to’.

• They lived in a little house beside the river.

Besides can be used like as well as (see here), when we add new information to what is already known.

• Besides literature, we have to study history and philosophy.
• Who was at the party besides Jack and the Bensons?

Besides can also be used informally as a discourse marker (see here) meaning ‘also’, ‘as well’, ‘in any case’. It is often used in speech to add a stronger, more conclusive argument to what has gone before. In this case, besides usually goes at the beginning of a clause.

• I don’t like those shoes; besides, they’re too expensive.
• It’s too late to go out now. Besides, it’s starting to rain.