Downloading files off the torrent network is a 2 step process. First you have to download the index file for the file you want to download. Then you feed the index file to the client and the client starts the original download.
Most of the time people download the index file and says that they downloaded the file, it was very fast and couldn’t open the file.
The index file or the torrent file contains information regarding the servers that track the original file that we want to download. Those servers are called trackers and they track the users who has got particular files. Our download client contacts the server to get a list of the users with the file and then connects directly to the users and downloads the files.
Some advanced technologies are now eliminating the need for centralised trackers. DHT is a technology in which peers communicate with each other propagating the data needed about the availability of the files with out a tracker.