ARC stands for Advance Reading Copy (though I've also seen a few variations, like Advanced Readers Copy) and basically looks like a paperback version of the hardcover first edition. A limited number are printed up, usually about six months or so before the book's release, and are then sent to reviewers, book bloggers, librarians and the such.
Here's an example of an ARC for The Sweetest Thing by Christina Mandelski.
The back, I think, is the most interesting. Besides the usual flap copy and author bio, it also gives you some technical info about the book (number of pages, price, ISBN etc.).
And there's a bit of information about the marketing plan behind the book, in this case: local author publicity, author blog tour, etc.
That's pretty much it! You may see ARC giveaways posted (sometimes by the author/publisher and sometimes by reviewers, etc.), so now you know what you'd be getting.
Also, thanks to Christina Mandelski for letting me make an example of her ARC. Her book comes out next week and if you're a fan of Ace of Cakes, romance, and heartfelt family stories, may I suggest you pick it up! (She's actually also running a giveaway here.)