Beside(without the s) is a preposition that meansnext 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 (with the s) can act as a preposition or a conjunctive adverb. Besidesmeans moreover, in addition, also, as well, in any casewhenit 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.
Besides means except when it acts as a preposition.
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.
Idioms: Beside oneself: very surprised; greatly agitated; in a state of extreme, uncontrolled emotion