What are seeds, peers, trackers, pieces in uTorrent?

Torrents are peer-to-peer file transfer protocol. That means, there is no central source for the files, people download files which are being uploaded by others (in real time).

Seeds: Number of peers who have already downloaded the files, and are currently uploading them.
Peers (or Leechers): A leecher is any peer that does not have the entire file and is downloading the file.
Trackers: A tracker is a server that keeps track of which seeds and peers are in the swarm (swarm: all peers sharing a torrent - uploading or downloading).
Pieces: This refers to the torrented files being divided up into equal specific sized pieces (e.g., 64kB, 128kB, 512kB, 1MB, 2MB or 4MB). The pieces are distributed in a random fashion among peers in order to optimize trading efficiency.