yup, yippy, yuh, are also other ways to say yes in more informal speech.

yepp

Nah, no, nope. :) You have your standard "yes", and you will use that about 95% of the time. The other variations are casual, and they also express the attitude of the speaker. You are communicating a lot more than a simple positive reply with "yeah/yep".

I second this. Depending on the tone as well, it can come off as rude, because the listener might think that you don't care about what is being said or discussed; even in informal situations.

Yeah, but be careful about using "yeah" or "yep" in a more formal scenario.