Hardcovers
Hardcovers have a sturdy, protective outside shell covered with paper, cloth, or leather. Typically, they are made with a thick binder board, like stiff cardboard, and have a flexible spine so an opened book can be laid flat. Hardcovers are more durable than other paperbacks and more costly to produce and purchase.
Paperbacks
Paperbacks are printed books with heavy paper stock on the outside. The pages are typically glued into the cover, so the spine must be broken to lay flat. They are lighter and cheaper than hardcovers but not as durable.
eBooks
Electronic books, or eBooks, are books in a digital format. This is a newer format characterized by which electronic device it uses (mobile, eReaders, desktops) and how it appears (some are static files, while others allow readers to change text sizes or highlight passages).
Audiobooks
Audiobooks have matured from books on tape and CDs to digital audio files. They require a published book, but they can open new audiences to authors and unique experiences for readers.