Sidebars are similar to a widget in that when used on a page, will show in the place they are used on (Head, body, footer, a section, inside a div, etc..), but are used as a container for other widgets. So you can imagine a Sidebar as a collection of widgets strapped together and shown in a similar manner as sidebars for other platforms (Think of it like the sidebar ads shown in google searches or the sidebars in Wordpress).