The section category list is a kind of menu list where all your sections and categories are in.
You can open and collapse them by pressing the Plus Sign.
When clicking on a Section or Category link you will go to that section or category and the page will show all articles in that section or category.
This module is usage free. You will only pay 1,50 euro (± $ 2,-) for the a private download and 5 euro (± 7 dollar) for a business download.
You can see a demo here.
There is a Major Joomla! 1.5 bug in where you cannot use SEF when opening a content section or category URLs. The URL to the Section and/or category list is wrong. Joomla! did not solve this problem yet. (bug tracker :http://snurl.eu/pxbwp)