When looking at a webpage remember that the page is broken down into five components: header, navigation, content, sidebars, and footers.  Here we are discussing the sidebars.

First what is a sidebar?

The sidebar is a narrow vertical column often jam-packed with lots of information about a website. Found on most WordPress sites, the sidebar is usually placed on the right or left-hand side of the web page, though in some cases, a site will feature two sidebars, one on each side of the main content where your posts are found.  Here is an example: