I posted a question about an ARC yesterday, but I have a new question....what exactly is an advanced readers copy, Besides it saying that on the back of the book? 

What is an ARC?

It is an unedited proof of a book for reviewers to read prior to publishing.

Usually ARCs are what reviewers get before the book is actually published. They can write the review and it will likely come out at the same time as the final published copy. The author usually gets an ARC too, to make sure that all of the edits they wanted from the galley changes have been incorporated.

my local SciFi bookclub group has several people who get ARC's and I when we meet, they always bring the latest stuff. It is pretty cool to read stuff before it is officially released :-)

ARC's have many different looks, some appear very similar to the mmpb edition except for maybe an ARC label on the spine and one of the covers.  Some are more tradesize with black and white cover, with or without a cover pic.  Saw another tradesize that was black printing on a solid goldenrod cardstock cover, again text only with no pic.  My UBS even had a large size spiral bound with photo type cover.

An ARC is not necessarily the finished product, there may be last minute changes to the contents.  They are usually distributed free to book dealers and critics for promotional and review purposes.  Most are labeled "Advanced Reader Copy, Not For Resale".