Seeders give, Leechers take. You know, like a leech... those little things in the water that own your skin.
With torrents, seeders are people that have 100% of the file. If a leecher downloads 100% and keeps uploading even after the download is finished, said leecher is now digivolved into a seeder.
In the event that there are two or more leechers (almost always) they will share with each other giving the seeder a bit of a break. Everybody wins.