A seed refers to a machine possessing 100% of the data. This includes any peer possessing 100% of the data or a web seed. A downloader that obtains 100% of the data becomes a seed.
Seeding solemnly refers to leaving a peer's connection available for downloaders to download from. Normally, a peer should seed more data than download. However, whether to seed or not, or how much to seed, is dependent on the availability of downloaders and the choice of the peer at the seeding end
Leech has two meanings. Often, leecher is synonymous with downloader : simply describing a peer or any client that does not have 100% of the data.
The term leech also refers to a peer (or peers) that has a negative effect on the swarm by having a very poor share ratio, downloading much more than they upload. Leeches may be on asymmetric Internet connections or do not leave their BitTorrent client open to seed the file after their download has completed. However, some leeches intentionally avoid uploading by using modified clients or excessively limiting their upload speed.