The .rar is a type of extension/file who have in it the original archive compressed, in other words, you open that .rar file and it are the archive.
Thats why the gcfs are in .rar because the rar compress the file and you can add a password and a description to the file too.
For install games in .rar firts install WinRAR, then do double click in the .rar, in a new window you will see the gcf/gcfs, move/extract the gcf/gcfs to the steampps and done.