1. Affirmative, yes, to convey agreeance.
2. Gives the subject a good characteristic, feeling or appearance.
"She is all yep" - The subject is in some way desirable, or has said or performed something desirable or agreeable to the speaker.
"Yep, that's a yep" - A double confirmation. Generally used after confirmation has been asked for on a particular subject.
It is also a way to say I agree with what you said
Ya and any other form of yes works
Yes doesn't work, it is just awkward
Sarah: Hey, are you going to that party tomorrow at brad's house
"Are you going to sit there all day? Yep...." Translation: "Yes, I most certainly am, and there is nothing you or anyone else can do about it."