ARCs (Advanced Reader Copies) are the book before final proofing. There might be a few typos or small changes made between the ARC and final book on release day, but nothing major. And, if you find a typo, you should TOTALLY point it out!
Seriously, help an author out and tell 'em about the typos!
In exchange for getting the book free and in advance of the release date, it'd be awesome if you'd leave an honest review on Amazon or Goodreads.
ARCs will be electronically delivered roughly 2 weeks prior to launch.