A 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 of getting a higher download speed.
A peer is one instance of a BitTorrent client running on a computer on the Internet to which other clients connect and transfer data. Usually a peer does not have the complete file, but only parts of it.
A leech is peer who has a negative effect on the swarm by having a very poor share ratio - in other words, downloading much more than they upload.