Both Windows and Mac OS X desktop windows include a sidebar by default. This column, which appears on the left side of each open window contains links to common folders, such as the Documents, Music, Pictures, and Downloads folders. It provides easy access to common locations on your computer, no matter what directory you are currently viewing.
Sidebars are also used by many software programs. For example, Web browsers such as Mozilla Firefox and Apple Safari use a sidebar to display bookmarks and bookmark folders. Email clients, like Microsoft Outlook and Mozilla Thunderbird display mail folders or new messages in the left sidebar, while the message content is displayed to the right. Word processors, such as Microsoft Word and Apple Pages, can be configured to display a list of all the pages in a document within the left sidebar.
Websites often use sidebars for navigation purposes. These sidebars typically contain links to the main sections of a website. If the sidebar is part of the website template, it provides easy access to any section of the website no matter what page you are viewing. When the primary navigation of a website is located in a sidebar, it may also be called a navigation bar.
NOTE: Windows Vista introduced a feature called the Windows Vista Sidebar, which was used to display desktop gadgets. In Windows 7, the Windows Vista Sidebar was renamed to Windows Desktop Gadgets.