How did someone RT one of my tweets without following me?

I am relatively new to twitter and I'm still in the process of figuring it out...I tweeted about something a little while ago and someone I've never met or even heard of re-tweeted it. How? Not that it's a bad thing, but I just want to know how they saw it if they're not following me and no one else re-tweeted it. Also, why doesn't twitter notify me when that happens? I get a text message but that's it.