WinRAR is an application which compresses file and folders into one compressed file. Which is basically another form of a Zip file.
Not really for organising files and folders, but very useful for making a backup of files, or for sending files by email, or for soting file that are rarely accessed but you don't want to delete, as the file size is compressed down as much as possible.
Here is WinRAR's website: http://www.rarlab.com/