The sidebars varies and depends on your choice of theme. Many WordPress themes support multiple sidebars also known as widget ready areas. Some support a header, single sidebar and footer area’s.
Sidebars play an important role in designing the layout of a WordPress website to display content other than the main articles of a website. For example, a short list of recent articles, recent comments, a list of pages, or popular articles on a website can easily be displayed across the entire site.
It can also be used to display advertisements from other websites. Users can drag and drop items into sidebars from Appearance » Widgets in the admin panel. Some WordPress themes come with highly configurable options to create dynamic layouts for different sections of a website, which allow users to add their own sidebars from a drag and drop interface.