It works like this. You go to their website, put in an ISBN, click "rent," pay, and you get books shipped to you within a couple days. You keep them for the whole semester, and then ship them back with a pre-paid label and no additional fees.

...And it works great. They have just about any textbook you can think of, and their rates range from "slightly better than buying used" to "fantastically cheap." It all depends on the demand for the textbook you're renting.

For example, I needed to get three textbooks this semester. Two ran for a bit over $100 new, and they were about 40 apiece to rent them for a semester from chegg. The third one was over 250 new, and I rented it for just over $20. So you see the prices range considerably. Occasionally it's not worth renting if the used price is close and you think you might want to keep the book, but chegg is usually quite a bit cheaper. It's saved me LOTS of money over the years. Oh, and if you're the environmental hippy type, they also plant a tree for every book you rent. Whoo?

One more thing: all the textbooks I've gotten have been of superb quality. About half of them were brand new, actually. Anyway, check them out if you think it might interest you.

EDIT: If you decide to rent from chegg, check a coupon site like retailmenot.com, or something. There's almost always a "10% off + free shipping" coupon going around.