Software packages in Ubuntu are divided into five categories:
Where does a package come from? Use packages.ubuntu.com to search.
When you install Ubuntu, this is the repository enabled by default. The main repository consists of only FOSS (free and open source software) that can be distributed freely without any restrictions. Software in this repository are fully supported by the Ubuntu developers. This is what Ubuntu will provide with security updates until your system reaches end of life.
Ubuntu tries to provide only free and open source software but that’s not always possible specially when it comes to supporting hardware. The restricted repositories consists of proprietary drivers.
This repository also consists free and open source software but Ubuntu doesn’t guarantee of regular security updates to software in this category. Software in this category are packaged and maintained by the community. The Universe repository has a vast amount of open source software and thus it enables you to have access to a huge number of software via apt package manager.
Software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu team. Also, please note that software in universe WILL NOT receive any review or updates from the Ubuntu security team.
Multiverse contains the software that are not FOSS. Due to licensing and legal issues, Ubuntu cannot enable this repository by default and cannot provide fix and updates. It’s up to you to decide if you want to use Multiverse repository, and check if you have the right to use the software.
Software from this repository is ENTIRELY UNSUPPORTED by the Ubuntu team, and may not be under a free licence. Please satisfy yourself as to your rights to use the software. Also, please note that software in multiverse WILL NOT receive any review or updates from the Ubuntu security team.
This repository consist of proprietary software packaged by Ubuntu for their partners. Earlier, Ubuntu used to provide Skype trough this repository.
This software is not part of Ubuntu, but is offered by Canonical and the respective vendors as a service to Ubuntu users.
Software from this repository may not have been tested as extensively as that contained in the main release, although it includes newer versions of some applications which may provide useful features. Also, please note that software in backports WILL NOT receive any review or updates from the Ubuntu security team.
Add/remove a repo
sudo add-apt-repository universe
sudo add-apt-repository -r universe