BitTorrent is a peer-to-peer (p2p) protocol that allows us to send big files around the world. It was developed by Bram Cohen and it allows companies to send big chunks of data without having too much trouble. It does not work like FTP does, whereby you download the file form one place and then wait for the server to push the data to you.
BitTorrent works by distributing parts of the file to different computers in the network and, during the time you download a file, you also allow parts of it to upload to the network. This way you won’t download the one file from a given server, you will download many different parts from many computers making sharing easy for the donor computer or server.
While p2p or bittorrent protocol is a legitimate one, there are certain parties that use it for pirating and thus gave it a bad name. Or maybe because it is such a good system, albeit a public one, in which anyone can see your computer on the network, bittorrent got to be used by digital pirates.