Torrent is a peer to peer network,there will not be any Centralized Servers to store and maintain data, it has small set of servers to find the seed and peer.

The user who gives data is known as seed,the user who downloads is known as peer.

If person1 has a music gets downloaded by person2. If person3 needs the same song which is available on both person1&2. The portions of data will be given by both person1&2 to person3.

Simply users who replicated data will acts as seeds.

You can see the amount of seed available for the download if its high you have fast download.if 0 your download never going to complete.

To know more about how torrent started