Historically, the main purpose of the sidebar has been to provide navigation assistance for the visitor - a function commonly needed to the present day. These navigation aids are designed to help people move about your site and find the information you want them to see. The list of navigation items includes Categories, Pages, Archives, and even the most recent posts. Another navigational tool you'll see in the sidebar is a search form to help people find what they are looking for on your site.
The first information in the standard WordPress installation is a list of Pages or Categories. Listing Pages helps the visitor find more information about your site, like About, Contact, Register, or Site Map.
The information displayed in the sidebar is controlled by your Theme's Template sidebar.php file.
An example usage of the Pages list template tag in your sidebar.php file might be: