Ansible modules are like functions or standalone scripts which run specific tasks idempotently. The return value of these are JSON string in stdout and input depends on the type of module. These are used by Ansible playbooks.
There are 2 types of modules in Ansible:
The core Ansible team is responsible for maintaining these modules thus these come with Ansible itself. The issues reported are fixed on priority than those in the “extras” repo.
The Ansible community maintains these modules so, for now, these are being shipped with Ansible but they might get discontinued in the future. These can be used but if there are any feature requests or issues they will be updated on low priority.
Now popular extra modules might enter into the core modules anytime. You may find these separate repos for these modules as ansible-modules-core and ansible-modules-extra respectively.