What happens if I make my Twitter account private?

I know it will allow me to determine who I want to follow me, but does it hide ALL of my tweets? For example, If I reply to a celebrity or athlete, will it not show up for them, since my account is private? If that is the case, I want to leave my account public. But if I can change it to private and still be able to tweet athletes and such, and have them be able to retweet me, then I'll make it private.