"Yes" to my mind is relatively formal whereas "yeah" is informal. Wiktionary
claims that "yep" originated as a variant of "yeah." I use it much less frequently than "yeah" (specifically, I think I personally use it only when I'm speaking informally and feel very confident about confirming something).
"Yas" is just a nonstandard form of "yes"; I think it's used when writing out the speech of people speaking some particular dialect(s) of English, but I'm not sure which one(s). I've also seen "yahs," which reminds me of how I see the way people speak in some parts of the northeastern US written out (but again, I'm not sure which accent/dialect that's supposed to represent, either).