- She sat beside her mother.
Beside can also mean compared with.
- You are quite tall beside your sister.
Besides can be used as a preposition with a similar meaning to as well as.
- Besides physics, we have to study chemistry and mathematics.
Besides can also be used as a discourse marker meaning also, as well. In this case it goes at the beginning of a clause.
- It is too late to go out now. Besides, it is starting to rain.