If you need to uncompress a .rar archive in Linux, you can download a program called unrar. If you are using Ubuntu or Debian, you can install unrar by opening a terminal and typing sudo apt-get install unrar. Then read the instructions by typing man unrar.
[UPDATE: to install unrar in Ubuntu you need the Multiverse Repository enabled.]
Basically, to unrar a file in Linux, just navigate to the directory where your rar archive is and type unrar x [filename.rar], replacing [filename.rar] with the name of your rar archive.
Also check out the great page over at cyberciti.biz on how to install and use rar files on Linux.