Bittorrent is a swarming downloading method which allows for large files to be distributed far quicker and more bandwidth efficiently than simply many users downloading from one server. When you use a torrent to download a file, you are ... contributing to the files distribution. And you are able to download from many soures providing the file download, not just one mirror. As you recieve sections of the file (which are individually verified) you will also start to send those sections of the file to other users who need them, similarly you will be receving the file from numerous other users. The more people who are attempting to download a file, the quicker it will spread.