What's a beta reader? 
This is the reader that comes before final edits. You read the book (in a PDF) and give your feedback on plot, pacing, character development, etc. I want your opinions on what to change here!

What's an ARC reader?
This is an Advanced Reader Copy that comes just before publication. You get to read the final book, for free (either e-book or paperback) and write a review on Amazon, Goodreads, B&N, or somewhere else! This is the absolute final copy, so no further changes will be made. You get a free book in exchange for a review!