BitTorrent is a tool for copying files from one machine to another. FTP punishes sites for being popular. Since all uploading is done from one place, a popular site needs big iron and big bandwidth. With BitTorrent, clients automatically mirror files they download, making the publisher's burden almost nothing.
Prior to version 6.0, BitTorrent was written in Python, and was free software. The source code for versions 4.x and 5.x are released under the BitTorrent Open Source License, a modified version of the Jabber Open Source License. Versions up to and including 3.4.2 were distributed under the MIT license. Version 5.3 was relicensed under the GPL.