FanFiction.net question: what is a 'beta reader'?

A beta is someone who will look over your story and help you with what ever needs to be done from gramaticle errors to helping you write the story itself. As you've said many authors will thank their betas and it makes sence considering the fact that the beta did something to help them. Another thing a beta might do is take over a story for you if you decide that you dont want it any more. they might have a good idea for the story and want to continue it so you might see something like "this story has been given to shady88" and that's what that means. also if a friend is a beta or just a friend sometimes you will be able to tell where their ideas come in because the "speech pattern" of the story will change. they are really awesome to have and a big help. Hope i've helped :3