Fixed Retainer or Removable?

I'm getting my braces off in about a year, I know it's a long time away, but what's the difference between a fixed and removable retainer? I have seen friends with a fixed one, you can't see it, it's behind their teeth.. and I think removable retainers are sooo ugly... Do you get a choice or is it up to the orthodontist?? What's the difference? PS I don't like taffy or carmel or anything like that... so that wouldn't be a problem anyway.