In the publishing business, a proof is an unedited paperback copy of a book printed solely for editors so they can a. get an idea as to what the book will look like in non-manuscript form (typeface, margins, line breaks and so on) and b. have a handy edition to mark up and make corrections to. Proofs are generally plainly bound with undecorated stiff paper covers of varying colors (though pale green seems to be excessively popular) and are refreshing in an odd way - proofs are the essence of information without the glitzy marketing gimmicks associated with modern printed matter. If it weren't for the typos, the printing errors and the fact that, just like everything else, ninety-five percent of everything eventually published is unreadable, they'd be a breath of fresh air.
While it's technically against the rules (NOT against the law) to sell proofs, most used bookstores of any significant size have proof sections. Because they're buggy and bland-looking they generally go for a buck or so and are a great way to feel out new literature without paying an arm and a leg for the shiny versions. Problem is, not all proofs for all books make it to the shelves and when they do they get snapped up rather quickly.
Some people are fanatical proof collectors to the point of waiting around the stacks of their used bookstore of choice until the proofs come out on any given week. Let me tell you, these people are scary - I was warned the first time I put out the proofs at my bookstore that a. the proof hunters aren't allowed to touch you, b. that they can only take the proofs off the shelf and not out of the box, ever, and that c. if they fight over the books, they get kicked to the curb. I assumed my coworkers were being dry or maudlin, but no - these people (predominantly middle-aged white women) watched over the boxes like hawks and grabbed at them the second they hit the boards. The only book people I know scarier than them (relative as this is) are the sci-fi hounds who insist on relating everything you try to introduce them to to Roger Zelazny.