There are numerous eBook file formats available. The two most popular are ePub and PDF but others exist such as Amazon's proprietary format. In addition, most new ebooks are 'protected' as they have Digital Rights Management (DRM) applied, which places restrictions on what you can do with your ebook.
File formats
ePub
ePub has reflowable text, which means that the display of the text should change easily to fit the screen of the particular device you use to read your eBooks, regardless of your individual settings. So, for example, if you increase the font size the text will rearrange itself so that you can continue to read as normal without having to scroll from left to right. This is ideal for straightforward text such as novels.
PDF
Unlike ePub, PDF is designed to replicate the look of the printed page, so the electronic version of the book looks exactly like the print edition. This is generally used for books where the layout is important: books for study, for example. You may still be able to increase the font size but the text may not reflow and the page breaks may remain, which can lead to an uncomfortable reading experience. A zooming feature, if available, is usually more effective to increase the text size, and when using a zoom feature the text will not reflow.
Digital Rights Management
Digital Rights Management (DRM) is used as a tool against piracy, but the biggest effect it has on you as a consumer is it restricts you to using certain hardware or software for reading your eBook. In the UK there are three main types of DRM: Adobe, Amazon and Apple. None of these formats are interchangeable: Adobe eBooks can only be read on eBook readers supported by Adobe, which is also the case for Amazon and Apple eBooks.
Adobe
Adobe eBooks are available in epub and pdf formats. The Adobe DRM means that you can only read your eBooks on platforms supported by Adobe. This includes Adobe Digital Editions eBook reading software for PC and Mac, as well as ebook reading devices such as the Sony range of Readers. You can buy eBooks for the Adobe platform from a wide range of eBook retailers, such as Waterstone's and WHSmith, and you can also borrow them from some public libraries.
Amazon
Amazon eBooks are in the 'AZW' or Kindle format. This is Amazon's proprietary format. You can only read Amazon's eBooks on the Amazon Kindle platform. This includes the Amazon Kindle eBook reading device, as well as Amazon Kindle software for your PC or Mac, or apps for your iPhone, iPad or Android smart phone. You can buy and download eBooks for the Amazon readers from the Amazon website.
Apple
Apple eBooks are in the 'iBook' format. This is essentially epub format with DRM applied which means you can only read eBooks from Apple on the Apple iBooks platform. You can download the iBooks app to your iPhone, iPod Touch or iPad from iTunes. You buy and download Apple eBooks from the iBookstore, which you access from the iBooks app.
Unprotected eBooks
There are many sources of unprotected eBooks (ebooks with no DRM applied): these are generally classics that are out of copyright. One of the most popular resources is Project Gutenburg. You can generally read these books on any eBook reader of your choice, provided the format is supported. Many eBook readers support a number of different unprotected eBook formats. The Sony Reader supports Adobe ePub and Adobe PDF, but will also support other unprotected formats. iBooks will support unprotected ePub files and PDF files.