The core components of an Ubuntu system and the vast majority of its applications are still installed from the classic package sources. Ubuntu organizes its packages in several repositories; the classification also provides information about the form in which the developers take care of the software and whether this software is free and “open source” or non-free (“proprietary”):
Contains all packages importent for a working Ubuntu system; therefore they are maintained by developers of Canonical, the compony behind Ubuntu, over a defined period of time. That includes technical support and security updates. This software is free and open source.
Contains most packages. However, this software is not maintained by Canonical itself, but by the community, on a voluntary basis. Again, this software is free and open source.
Even a basically free Linux distribution such as Ubuntu does not come without unfree components. These may be, for example, the firmware for hardware components such as WLAN chips or drivers for graphics cards – so-called “blobs”, whose manufacturers do not supply source code, but only binary data. Unlike Debian, which has strict policies, Ubuntu makes installing such software from Restricted easy, but you have to trust the manufacturers.
Contains non-free packages, that is, the software is under a license that is not compatible with that of Ubuntu, and therefore is not maintained by the Ubuntu developers. Installation at your own risk.
Discontinued. Here Canonical used to provide commercial software like Skype or Adobe Flash. Those who could not do without such programs could activate the Partner repository; then install and update these programs as usual by package management.