The pages within an Informl wiki are organized into books. One of the books is designated as the "premiere" book, which is basically the default book for the site. Each book can have its own stylesheet, and RSS feeds are available on a per-book basis (per-page feeds and submissions feeds are also available). The informl project site, informl.folklogic.net, has four books: Documentation, the Sun King Demo, the Wine Store Demo, and the Sandbox.
One of the more unusual features of Informl is that its pages can contain form elements. When a page contains a form, Informl creates a database table to store the data submitted through the form. When you want to refer to the data associated with a page, you do so using the page's name. (If you go and look in the SQL database, you will find tables whose names include the names of the pages that contain forms.)
Every form created with informl has four built-in fields: id, created_at, updated_at, and global_id, plus whatever fields are defined by the content of the page. Each of these fields corresponds to a column in the database table for that page. The names of the fields are supplied by the wiki markup.
For example, in the Sun King demo book on the informl project site there is a page for requesting an audience with the king. This page contains this markup defining a text area (3 lines of 20 characters each):
Reason for request:: [20x3<>]
Reason for request is one of the fields on that page. You won't need to know this unless you want to access the database from outside informl, but there's a corresponding text column in the database named Reason_for_request.
See the discussion on referencing for more information about how Informl uses the names of books, pages, and fields.