well yay means cheering kinda like what you do when ur fav sport team wins a game. and boo is when you are calling somebody onstage or something bad. Like when your most unfav sport team wins a game.
EDIT: oh and sometimes boo is what people write when they scare people. Like if somebody dressed as a ghost and jumps out of a bush in front of you saying "BOO!" it will scare you and stuff. So that is the 2nd definiton of BOO.