Riba

wa alaykum as salaam wa rahmatullahi wa barakatuhu!

Thank you for your question! As for what constitutes riba, it is basically any unlawful increase gained as profit from a transaction. It is also the sale of an item before taking possession of it.

This would include those arbitrary fees charged by banks. That withstanding, interest from any bank--Muslim or otherwise--is considered impermissible. 

And Allah knows best!

IRS