Backdrop CMS is a simple, lightweight, and easy to use Content Management System for building professional websites.
Backdrop is a Content Management System. A CMS is a database driven website that allows people to log in, and if they have permission, it allows them to "edit" nearly any page on the website. Backdrop can be used to create a wide variety of websites ranging from simple personal blog sites, to more comprehensive non-profit, corporate, or government sites.
Backdrop is a tool for building websites. The core Backdrop CMS package aims to include many useful features, but only those that are necessary for the majority of sites using it. Backdrop's world of Add-Ons can be used to meet less common needs. Backdrop can be easily extended with the addition of modules (that change the way it works), themes (that change the way it looks), and layouts (that change where content can be placed on a page).
Backdrop CMS is the Drupal fork. Backdrop's founders and early contributors saw the huge changes coming to Drupal version 8, and watched as many in the Drupal community feared they wouldn't be able (or willing) to make the jump from version 7. Backdrop hopes to fill the gap by providing an easy upgrade from Drupal 7, while still adding new features and making key improvements.