Who is Khodorkovsky and what did he do?

I know this matter is from a while back, but I'd like to know more. Who is Mikhail Khodorkovsky and what did he do to get himself thrown in jail? And why is America and Canada freaking out about it? And why do people keep saying Khodorkovsky should be pardoned? Because some people say he should not. Sorry if I seem naive about the way Russia works, I've never lived there, though I am Russian.