Twitter - How does it work?

[acss]

I was wondering if someone could tell me how twitter works?
I know you post status updates, but that's as far as I know...

What I want to find out is, if you write a message to someone else, do they see it on their wall? or only if they're following you? and do other people also see it?

Can I do different settings for different groups like on Facebook? Ideally I want to know if I can write to someone without my whole friends group knowing?

Thanks...and apologies for putting so many questions in one!