Beside (without the s) is a preposition that means next to, along side of, at the side of
- "The tree that is beside the running water is fresher and gives more fruit" ~ Saint Teresa
- The little girl was sitting beside her mother.
- A man should stand beside his woman.
- Who is the big girl sitting beside Anne?
Besides means moreover, in addition, also, as well, in any case when it acts as a conjunctive adverb.
- Besides chocolate, you will also need a second flavoring in the ice-cream.
- I don't like those earrings; besides, they're t costly.
- I'm to exhausted to type the report tonight; besides, it's not due until next week.
- He is naive, immature and irritable; besides, he is too young for you.
- No one besides her mother knew her secret.
- No one besides the trainer could control the dog.
- Nobody besides Sam can deal with her
- No one besides Sam would use such an offending tone.
Beside oneself: very surprised; greatly agitated; in a state of extreme, uncontrolled emotion
- He was beside himself with anger.
- Your idea is beside the point.
