Hai is the word they use for yes.
It gets used for a lot though. Like here we'd be going, "uhuh" while someone is talking to show we understand. They'll say, hai, for that.
I've seen that word range from formal, "yes, sir." to very informal like, "uhuh."
So you can think of it as similiar to our yes.
And from what I've seen they do use it alot.
Ruby Chan, if you want to see some really sugoi interviews, you should get the D.N. Angel dvds. They have interviews with the guy who does Daisuke and the guy who sings the theme song. It's good to watch both guy and girl interviews, because guys will use different words than girls will. So, you can become familiar with which words those are.
I like how they say sugoi alot. For some reason I find that really kawaii!
I'm waiting for Fruits Basket 3 to come right now, I think it has the interview with Seki on it!