I found this and thought it was worth repeating:
“Main” primarily consists of supported open-source software, but also includes proprietary firmware and fonts. This is roughly equivalent to Debian’s code. The proprietary programs, however, must be governed by a license that makes them free to distribute.
Ubuntu also supports some proprietary software that is not part of the kernel in a component called “Restricted.” This is software, like the NVIDA and ATI graphics card drivers, that can not be freely distributed. The Ubuntu developers have also made special arrangements in a subset of Restricted known as “Commercial.” These are proprietary programs, such as the Opera web browser and the RealPlayer media player, which Ubuntu has received permission to distribute to Ubuntu users.
Finally, Ubuntu includes the “Universe” and “Multiverse” categories. These contain non-supported open-source software, and non-supported proprietary programs, respectively.
Source: “FSF debuts fully-free Ubuntu/Debian variant“, emphasis added.