CiviCRM is a cross-platform Constituent Relationships Management tool that works with Backdrop CMS (as well as several other CMSes). It is quite large and complex, includes several modules, and the best way to learn about it is on the CiviCRM.org website. In brief, it handles:
- Contacts
- Events
- Membership
- Mailings
- Donations
and much, much more.
CiviCRM is not hosted on GitHub like other Backdrop modules; instead you can and should download it directly from the CiviCRM website.
CiviCRM has its own extensive documentation. Most of CiviCRM and its documentation is CMS-agnostic, with platform-specific bits where relevant. Some of the Backdrop-specific documentation pages are: