I can't stand it when people misuse yeah/yay/yea.

Here's a little primer:

Yay! = A synonym for Hooray! (Yay, I got into college!)
Yea = An affirmative vote, antonym of Nay, or Biblical sentence starter (All for: Yea. All against: Nay; Yea, and in the fourth year it came to pass...)
Yeah - A casual synonym for "yes" (Yeah, it's okay I guess...)
Yah - Bill and Ted.     ("Ted, are you alright?" "Yah!" "Excellent!" )
Yaw - Pivoting around the Z axis   (These jets adjust the spaceships yaw since there is no air in space on which to bank.)
Yar - Pirate for yes.  ("Cap'n, should we give chase to them scurvy dogs?" "Yar!")
Yar - Pirate for no.  ("Cap'n, should we give chase to them scurvy dogs?" "Yar.")