Often it is necessary to extract files. zip and tar.gz files can be opened directly in Ubuntu (and other flavors of Linux), but rar is not supported by default. You should install and use unrar to extract rar files.
% sudo apt-get install unrar
Two useful and probably most used command are:
% unrar e your-file-name.rar
Extracts the rar file in the current directory.
% unrar l your-file-name.rar
Lists the content of the rar file.