A book is basically a collection of articles (pages or chapters) that are bound together as a group. Another significant aspect of a book is that it has its own navigation. Each page contains links to pages Previous to, after and above itself.
A book is a handy tool to use when you want to create a huge set of information individually or collaboratively. A book might hence be throught of as a collection of wikipedia articles that are liked through navigation. For example, the help topic you are reading is a part of 'What is a..' book.