Ubuntu has the following repositories:
- Main - Includes all the packages that are installed by default, such as the Linux kernel, the Gnome desktop, etc. These packages are officially supported (meaning they receive security updates during the support period of the Ubuntu release, which is 18 months for most releases, and 3 years for a Long-Term Support release). Only free software can appear in Main
- Restricted - These are packages that are not available under a completely free license - in other words, that don't qualify as free software. They may in many cases be necessary components, such as device drivers, where the Ubuntu developers have no alternative and not making them available would mean people could not use Ubuntu. They are clearly separated so that people who would prefer not to use them can remove them easily.
- Universe - Packages maintained by the Ubuntu community. This generally consists of applications people may want to use, but compiled against the libraries and using the tools present in Main so it should install and work well with the software in Main. However, unlike with Main there are no guarantees. However, like Main, Universe consists entirely of free software.
- Multiverse - Similar to Universe, but contains non-free software only, and is separate from Universe for this reason.
- Backports - This provides more up-to-date versions of packages alredy available elsewhere. Normally, during the life of an Ubuntu release, you will get minor version upgrades, such as bugfixes, but major upgrades, such as going from version 1.1 to version 2, will not be available. Backports includes more recent, but less well-tested versions of popular applications so those who like to live on the bleeding edge or just can't live without a brand new feature can have it without having to resort to either compiling from source themselves or installing from a .tar package (which can be a pain compared with using .deb packages). Refer to this link for more information.