RAR is an archive file format that supports multiple file spanning, data compression and error recovery. As a matter of fact even in Ubuntu 10.10 unrar is not pre-installed because of copyright reasons (read: RAR support is not freely available).
This problem arises when, trying to open a rar archive, you get an error message like this:
Cannot open «rar_archive.rar» Archive type not supported.
So, before uncompressing a rar archive in Ubuntu you will need to install an application called unrar (http://packages.ubuntu.com/karmic/unrar).
To install unrar go to Terminal and fire the following command:
sudo apt-get install unrar
Now, in your terminal session, navigate to where your RAR file is stored and run this command to extract the archive:
unrar x rar_archive.rar
.. or just run this other command to list files inside your rar archive:
unrar l rar_archive.rar
I hope this helped you !