I follow some people on Twitter, that don't follow me.
Can THEY see my tweets, even if they don't follow me?
There are two answers to this:
Yes. All they have to do is go to your page
No. If they go to your page
If they then try to follow you, you'll get a notification that someone wants to follow you and you'll have to approve the request before they see your tweets.
It really depends on the access restrictions on your tweets. If your account is public (as is by default), anyone can see your tweets, irrespective of whether they are following you.