An e-book (electronic book) is any type of publication in digital form, published for an electronic device. A dedicated e-reader (electronic reader) is one such device and allows you to read a book stored in the e-reader. The books themselves can be printed books made into electronic form or books which were developed specifically for e-readers. So basically, an e-book is an electronic form of a printed book read on an e-reader such as a Kindle, Nook or iPad.
To the average reader, you just turn on your e-reader and read your book. To a publisher however, it is not so simple. First, you can’t just take a book and put it on an e-reader. The information has to be formatted in a form that can be read by the e-reader. To complicate matters, most e-readers demand their own specific formatting to fit their reader. So a book that can be read on the Kindle, may not be able to be read by an iPad because the formatting is different.
First, a little back ground on e-readers. When you are reading a book, unlike a static page of a printed book, the text on an e-reader actually flows to fit the screen. This is called reflowable text (like word wrap). It allows the text to fit on whatever screen you are viewing at the time. So a page you might read on your iPhone would be different than the same page you view on your iPad. The words flow to fit the size of your screen.
Another difference from the printed page is that the actual text you see on your e-reader is different than the styling. Both are kept in different files. When you call up a book, your text is loaded first, and then styled. Since this process for the different types of e-readers may differ, the formatting will be different. While you may see nice charts in one e-reader, the same information may show up as gobbly gook in another. It is this formatting that makes your reading easy and clear.
And it is this formatting which makes it difficult to read the same material on different e-readers. The most used format is .epub which tends to be widely supported. For example, .epub is used on Apple’s iOS devices (iPhones, iTouchs, iPads), Sony Reader, B&N's Nook and many other devices. Devices that can’t read it directly have special adaptations to allow their e-readers to use .epub. In addition, some formats combine or add extensions (like an .html extension), so that not only text can be read, but also enables a limited audio and video within the text.
Amazon’s format for the Kindle is .azw, better known as mobi. It is proprietary and does not have integrated support for the .epub format. But because your books are downloaded automatically onto the Kindle, you don’t really see the type of files so it doesn’t tend to be an issue unless you want to read your Kindle book directly on your iPad (without a Kindle app). Unfortunately, no matter which format you use, it is not as simple as just putting your Word document through some sort of .epub or .mobi filter to come out the other side with a readable e-book. (For example, Our Little Books uses one of our design editors to prepare our little books for all the different forms for e-book publishing of the printed little books.)
Why is this important? Because if you are writing an e-book and want to get it into Amazon or in the Apple store, it has to be formatted in the correct manner to not only be read by your readers, but to be accepted by the stores for sale. After all, you certainly want your readers to be able to read the wonderful book you have just written!