When your need to put a lot of information into your interface, you may find out that your window has to grow intolerably big in order to hold all the information. Having a window that's 10000 pixels wide and 5000 pixels high doesn't seem to be the perfect solution. Of course, you can ``chop up'' your big window into a set of smaller dialog boxes, but the user will most likely find it impossible to manage 20 different dialog boxes on their desktop.
The TixNoteBook (fig 2-1) widget comes into rescue. It allows you to pack a large interface into manageable ``pages'' using a notebook metaphor: it contains multiple pages with anything you want on them, displays one at a time, and attaches a tab to each page so the user can bring it forward with a single click on the tab.