A seed is a user who has 100% of the file or folder and is currently still uploading the file to others. Peers would not have 100% of the file but are currently downloading more parts and uploading the parts they have to other peers. Leechers is a term for someone who is downloading the file, but has either cracked their upload or limited it so much they are barely uploading at all. These users are called leechers because they just grab their files and go. They slow down the overall file transfer