As part of the recent "Backdrop LIVE" virtual conference, we hosted a session giving a top-level overview of CiviCRM functionality and integration with Backdrop CMS.
Backdrop CMS is an open source CMS that provides a powerful, flexible, scalable base with a user-friendly interface and is our go-to platform for web development. It has a strong feature-set, including a user-friendly layout system, custom content types, custom fields, robust taxonomy/tagging, configuration management, customizable views, and granular permissions. It integrates deeply with CiviCRM to provide donor and membership management on-site.
The project has a specific focus on affordability over the long term and continuously improving the user experience. (Backdrop forked from the Drupal project in January 2015).