Nice article about torrents here: http://en.wikipedia.org/wiki/Bittorrent
Definition of seed from article:
A Seed is used to refer to a peer who has 100% of the data. When a leech obtains 100% of the data, that peer by definition becomes a Seed.
Seeding refers to leaving a peer's connection available for other peers, i.e. leeches, 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 leeches and the choice of the peer at the seeding end.