The following is from the: Techencyclopedia Web site / techweb.com
(Electronic-book) A handheld device that is specialized for displaying electronic versions of books. Like its printed counterpart, an e-book lets you set bookmarks and annotate in the margins.
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Evolution
However this has evolved and eliminated the need of a handheld device. Your computer can house a program such as (Open eBook, Microsoft Reader, Adobe Acrobat) and other eBook reader utilities. Each of these programs pretty much supports it's own platform technology and not the format of the other products. In other words you will need to use the appropriate reader with each eBook.
For example, if you download an eBook created in the PDF format, you will need Adobe Acrobat to read it. If you download an eBook in the LIT format, you will need Microsoft Reader to read it and so on.
eBook Vs Text Document
Someone may ask the question what is the difference in using a eBook format and merely typing a book in a word processing that creates rich text or plain text documents? The answer would be that of authentication, copyright/digital-rights management, and compatibility.
Advantages
For starters, a MS Word, Corell Word, Note Pad, Word Pad, or other word processor software generated text can be opened in many different applications and altered. This will create problems with the integrity of the work which must be maintained to insure copyrights/digital rights. Secondly, Only someone must have a software application (sometimes very expensive) capable of opening the file. Yes, this can be accomplished with "viewers" (usually provided for free) created by the software company, however this is dependent on the company making the viewer available.
Third, most "viewers" are pretty naked when it comes to features, of course the manufacturer doesn't want to give you a great deal of free features which will hurt sales of the full software application. Forth, a software application must be able to run on the OS (operating system) platform. If a person running Linux, or Macintosh wants to read a document created using a word processing application that runs on a Windows OS, they will need a special utility to open that document. Whether or not the utility is available will depend on many factors that are constantly changing.
Last and most important, software applications designed to generate eBooks usually allow you to create your eBook in many different formats "text, html, doc" and other, and convert them into a "reader supported" format. This creates reusability and flexibility. An author/publisher can take their documents created in all different types of formats and covert them into one format that is cross-platform capable. Most eBook formats can be run on multiple OS platforms and are designed to provide the user with a myriad of features that facilitate the eBook experience.