BitTorrent is a relatively new sort of p2p network, more information and background can be found here.
In order to download files you need a BitTorrent download client. Here are the ones we recommend:
Someone is called a seeder when he or she is uploading a file, a peer/leecher is someone who has yet to finish the download.
There is loads of information available on the internet about this subject, do a search on google.