E-books (or ebooks or eBooks) are ''electronic books.'' They're downloadable files, available in a variety of formats, that can be read on a computer or on various readers, such as the Palm or RocketBook. Some formats may require specific hardware, while others are cross-platform. Some offer extras such as cover art, search capabilities, etc.

There's plenty of information to be found on the Web; the publishers can tell you far more than I can!