A print book’s cover is made to wrap around the entire book. It covers the front, the “spine" and the rear. Print books are usually laid out at a size ratio of 2:3, with 2 being the “short side" and 3 being the “long side.”
EBook covers only show the front cover. No spine or rear cover is required, or even desirable. This is why eBook cover designs save you so much money! Remember that ebooks are shaped differently than print, and range in ratio from 3:4 to 1:1.7.
More importantly, think of the "job" that a rear cover does for you, with regard to marketing. A rear cover has blurbs (praise from other authors or readers), a book description or a "hook." All of this is there in order to induce someone to buy your book. In an ebook, the rear cover wouldn't be displayed until after the reader already had purchased the book, read it, and completed it, so the marketing "blitz" of the rear cover is a bit unnecessary at that point. Instead, Amazon, B&N, etc., give you the book description pages, Editorial Reviews section, etc., in which to put this information.
For the latest updates on the "perfect" ebook cover sizes, please see our FAQ Entry, What is the Correct Cover Size for Amazon eBooks?, which you can click through to by clicking that link. We keep this article updated as each new cover size is announced.