I'm a bit confused...

What are these 'ninja'? Some of them seem like normal humans... but others are weird monsters... or have their powers transformed them into monsters?
They all belong to a 'clan' but I guess that doesn't mean they're blood-related, so it's more like a club? a gang?
I'm guessing that in Japan all of this would make perfect sense, drawing on familiar tropes and famous stories, but lacking that I'm left with lots of questions.

Thanks for any thoughts on this.