Most of the software available for Ubuntu is free, open-source software. This software is free for anyone to install and use, and people can modify the software and redistribute it if they like. Ubuntu is built from this type of software.

Non-free software is software which is not freely redistributable or modifiable. This makes it difficult for the Ubuntu developers to improve the software and correct problems, so it is normally recommended that you use free software instead.

Restricted software is software which has restrictions on its use, preventing it from being classed as free software. Non-free software is a type of restricted software, where the restrictions are due to the software having a non-free license. Other reasons for software being classed as restricted include legal issues (use of some types of software is illegal in some countries) and patent issues (some software requires a patent license to be used legally).

In some cases, restricted software is the only option. Such cases include software for the playback of certain audio and video formats, some fonts and drivers for some graphics cards.

You should be warned by the package manager when you try to install restricted software. If the restricted software cannot be used legally in your country then there is little you can do; you should not install the software. If the software is restricted simply because it is non-free, you may choose to use it (for example, in the case of graphics card drivers). Be aware that most restricted software is not supported in Ubuntu, and problems with such software often cannot be corrected by Ubuntu developers.