As in Drupal and Wordpress, an enormous amount of functionality is provided by contributed modules (often referred to by the shorthand “contrib”).
For people upgrading to Backdrop from Drupal 7, the vast majority of the top 100 Drupal 7 modules have either been ported to Backdrop or are already present in Backdrop core.
Existing Modules
There are over 1000 modules available for Backdrop, with more being added each week. There are several resources to find the modules you may need:
The Backdrop modules page has stable releases.
The official distribution channel for Backdrop modules is the Backdrop modules page on backdropcms.org. This lists all modules that have an official release (or pre-release). Try some keywords to find modules that address particular needs. To find ports of Drupal 7 modules, use the name of the module.
The Github backdrop-contrib pepository contains all projects.
You can find all of the above, plus other modules, themes, and layout templates that are still in development in the Backdrop Contrib Github repository. Projects that haven’t been released yet are likely not ready for production use. If you find something that you would like to use, please test it in a safe enviornment (like one of our demo sandbox sites). If it looks ready to use, please drop a note to the developer expressing your interest (or possibly your willingness to help bring it to production).
Developing Modules
If you can’t find a module that does what you want in the above, there is the option of developing it yourself—whether porting an existing Drupal 7 to Backdrop, or creating a module with new functionality ab initio.
Porting an existing Drupal 7 module
Documentation for converting Drupal 7 modules can be found here. Before starting a conversion, you should check the Drupal 7 module project issue queue to see if a Backdrop port has been started (look for the tag “backdrop-port”); if you don’t see one (and you don’t see a port in the Github backdrop-contrib repository), then it’s probably safe to start the port.
Creating a new module
You may also be able create a new module with some PHP and Drupal 7 or Backdrop CMS knowledge. Documentation for developing new contributed modules can be found here.