Metadata file which is very small and contains information about the target files you wish to download. It also contains the exact coordinates of the target files.
leech is synonymous with downloader: used simply to describe a peer or any client that does not have 100% of the data.
A Seeder is used to refer to a peer who already has 100% of the data. When a leech obtains 100% of the data when the target file has been downloaded), that peer by definition becomes a Seeder.Seeding refers to leaving a peer's connection available for other peers, i.e. leechers, to download from. Normally, a peer should seed more data than leech. However, whether to seed or not, or how much to seed, is dependent on the availability of leechers and the choice of the peer at the seeding end.
A tracker is a server that keeps track of which seeds and peers are in the swarm. Clients report information to the tracker periodically and in exchange, receive information about other clients to which they can connect. The tracker is not directly involved in the data transfer and does not have a copy of the file.
For a complete listings of terms you can visit Wikipedia.org
Also see What is a torrent file for more information.