1. Main – Free and Open-Source Software Maintained by Canonical
The Main repository is the largest one. It includes all core packages as well as all the free and open-source packages from the default Ubuntu installation. All the software in the Main repo is free and open-source – no exceptions here.
All the software in the Main repo is maintained by Canonical, the parent company of Ubuntu. This means the package has been tested for compliance before inclusion in the build. It also means that security updates and critical bug fixes for the package will be provided by Canonical for the lifetime of this particular Ubuntu release (three or five years or more – this varies among releases).
2. Universe – Free and Open-Source Software Maintained by the Community
Since (fortunately) there are so many free and open-source apps out there, it’s not possible to include them all in one single software distribution, even if it is Ubuntu. This is why any packages that are both free and open-source that didn’t make it to the Main repository go into Universe.
Similarly to the software in the Main repo, all the stuff in Universe is both free and open source – again, no exceptions here. However, unlike the Main repo, the software in the Universe repo is not managed by Canonical but by the huge community of open-source enthusiasts and volunteers. This doesn’t mean the software is second class – it means the software for one reason or another wasn’t included in the main installation and that Canonical has no responsibility for dealing with its bugs and updates – the Community does. The good news is that since this is open-source software, if there is a serious bug or a need for a patch/update, it’s usually provided by somebody, most often the author/maintainer of the app itself.
3. Restricted – Proprietary Software
For many die-hard open-source fans, it’s hard to accept that Ubuntu comes with proprietary software too, but since this software provides functionality with no open-source alternative, proprietary software is present in many distros, Ubuntu included.
At present the list of proprietary software included in the default Ubuntu installation includes drivers only, such as WiFi drivers or video card drivers. Since these drivers are vital to the normal function of the operating system, they are officially maintained by Canonical. This means Canonical is obliged to provide updates and fixes for any restricted software in this repo for the lifetime of the release. The problem here is that since this software is close-sourced, Canonical depends on the willingness of the driver vendor to make fixes/updates.
4. Multiverse – Software Restricted by Copyright or Legal Issues
The last repository, Multiverse, is the place where anything else goes. This includes mostly proprietary software such as codecs, Skype plugins, Adobe Flash, etc. Open-source software with legal issues or with an unknown license can also be found here.
Canonical has no responsibility for this software; it’s maintained by the community. Since this software isn’t tested and supported, it’s best if you install apps from here only if you are 100 per cent certain in them.
In a Nutshell
The differences between the four repositories become very clear when you compare them like this: If you memorize this table, any time you wonder what the specifics of a repo type are, you will be able to easily figure them out.
