BitTorrent is a P2P system that makes transfering large files (or groups of files) amongst a large group of people easy, fast and efficient. The BitTorrent network is set up in a way that is a little different than a normal P2P network. With this network, you really don't do any searching for files that the other users have using the client as you would with traditional clients. Rather, you go to websites that have lists of recently released files.
The websites listing the torrents are called indexing sites and have become very popular. Their popularity has come at a cost however, with the MPAA and law enforcement agencies shutting them down. There are no shortage of sites to go to so long as you know which are currently being used, as they come and go so quickly these days. The files that you get all come as a package called an archive, and to receive them you must first get a .torrent file (don't worry, that's what you went to these websites for). Once you have downloaded the .torrent file, your BitTorrent client will kick in and connect to a managing computer that then connects you to others who have the files.
BitTorrent is a P2P system that uses a central location to manage users' downloads. The central location is a tracker that is connected to when you download and launch a .torrent file. The tracker keeps track of all the people who have the file (both partially and completely) and connects users to each other for downloading and uploading.
The BitTorrent network is impressive mostly for the speed and reliability with which you get files that have been recently released. As long as a .torrent file has enough people sharing then you can be assured that it will start quickly and maintain a good download speed. Most indexing sites have a system of listing the number of Seeders and Leechers for a particular archive.
What is a seeder?
A seed(er) is a client on the BT network that has a complete copy of a particular archive. For any archive to work, there must be at least one seed to download from originally. Sometimes under certain circumstances, there may be no one seeder but enough people with all the parts to make up the whole archive, this is called a distributed copy. It is HIGHLY recommended that once you have gotten an archive you leave the BT client running for at least the amount of time that it took you to download the archive to help ensure that others will also be able to get it. Share and Share alike!
What is a leecher?
A leech(er) is a client on the BT network that does not have a complete copy of a particular archive yet. When any new client begins downloading an archive, they are a leecher until they have finished downloading the entire archive and then become a seeder. The name 'leecher' here is an unfortunate use that has become too commonly used even though it is really not applicable to what the meaning is. A leecher normally means someone who downloads without uploading (takes but does not give.) But here, a leecher is part of the network and is uploading as well, many times more KB than they download. But hey, it's worth the new meaning once you appreciate the vast resources that BT brings to you.
What is a .torrent file?
To download something with BitTorrent, you must have a .torrent file. This file contains in it a location that tells the BitTorrent client where to go to find the tracker that manages the uploading and downloading of the archive. An archive is a complete set for downloading which may include one file or many files. The one .torrent file contains the archive information also. To download on this network it is simply a matter of clicking on the .torrent file in your web browser, then the BitTorrent client kicks in and asks where you'd like to save the archive. That's it, from there you just sit back and watch the client work it's magic.