A seed refers to uploader. Someone who is uploading the torrent and it's contents.
A peer is a user; you, me or whomever connects to download the torrent.
A leecher is a user who only downloads and doesn't upload, this causes bad share ratio and the file becomes less available because there isn't any multiple sources to download from which is required when you download using torrents.
(If you connect to the tracker, you become a peer)
Torrents is a well known method of filesharing.
You can setup a torrent, upload it to a tracker and download it from the network.
People (peers) connect to your torrent and a list of available sources to download the content of the torrent becomes available, this is called connecting to the DHT (Decentralized Hub Network) network and swarm (current group of users uploading/downloading the same torrent).
This depends on how many users are 'seeding', or, uploading the torrent back onto the tracker.
The more the torrent is re-uploaded the more copies of the file become available and the quicker you can connect to multiple locations (other users) and download the torrent, because the content is readily available.