BitTorrent is a peer-to-peer file sharing (P2P) communications protocol. BitTorrent is a method of distributing large amounts of data widely without the original distributor incurring the entire costs of hardware, hosting and bandwidth resources. Instead, when data is distributed using the BitTorrent protocol, each recipient supplies pieces of the data to newer recipients, reducing the cost and burden on any given individual source, providing redundancy against system problems, and reducing dependence on the original distributor. (Source: Wikipedia)
Leecher – Client without complete file or file group. After leecher acquires complete file or file group it becomes a seeder.
Peer – Client participating in p2p data exchange process.
Seeder – seeder is a peer that has a complete copy of the torrent and still offers it for upload. The more seeders there are, the better the chances are for completion of the file.
Swarm - Together, all peers (including seeders) sharing a torrent are called a swarm.
Torrent – Can mean either a .torrent file with metadata about shared files or files themselves. .torrent files hold tracker addresses, file list and file sizes.
Tracker – A tracker is a server that keeps track of which seeds and peers are in the swarm.