A leecher is a user who downloads more than they upload. A leecher might not share any data after downloading the full file, or might limit their upload speed to a very low level. This behavior reduces the availability and speed of the file for other users, and is considered unethical by the torrent community.
To summarize, peers are users who contribute to the torrent network by sharing data, while leechers are users who take more than they give and harm the network. If you use torrents, you should try to be a good peer and avoid being a leecher.