Seeders are the peers who have completed downloading the file 100%
A seed is a peer that has a complete copy of the torrent’s contents and keeps uploading it. That can be done by leaving the task running after completing the download. The more seeders there are, the better the chances of getting a higher download speed.
Peers are people who are downloading the file off the seeders, they also upload a parts of the file too.
Running torrent clients that transfers data to and from other clients, is called a peer. A peer does not have the complete file; only parts of it.
Sometimes peers are also called leeches, which mean downloaders.
some information courtesy of ABIT.CDA11