Probably the best way to explain is to point you to it.
http://en.wikibooks.org/wiki/Main_Page
Some of the key features are:
* The ability to easily export/import entire books, modules or pages between mediawikis
http://en.wikibooks.org/wiki/Wikibooks:Database_download
* Ability to create print version
http://en.wikibooks.org/wiki/Wikibooks:Print_versions
* Layouts suited to book reading
exampe:
http://en.wikibooks.org/wiki/Cookbook:Texas_Easy_Chili
Have I better explained?
Lars