Linux-Ubuntu comes with native zip file support (zip and unzip). Good. Unfortunately, uncompressing RAR files (or unrar-ing files) is not handled by Ubuntu. But it’s not a big problem: just install the unrar package and you’re ok. To install unrar, start a terminal and type the following command:
$ sudo apt-get install unrar
Now you can unrar files in Nautilus just like you unzip files: by using Extract Here in the contextual menu.