torrents use trackers, which are embedded into the torrent file. Torrent search engines can search for these trackers and anyone seeding them with matching trackers can pull off the seeder. Private torrent sites work the same way, except they only use private torrents from their tracker.
Napster had servers, that connected to to someone, so in a way it was kind of like a P2P gateway I suppose. That is how it got shut down.
Newer P2P apps like torrents, really can not be traced or tracked by ISP, unless you are using ungodly amounts of bandwidth, or someone is monitoring you by IP/MAC address. However, a government agency would have to supena the ISP for rights to collect information about its users. Most ISPs won't just hand that information over, they want to respect their clients privacy.
Since there are no centralized servers to connect to torrents it is harder to shut down. Also, torrent sites are generally just search engines. In fact you can just use google to search for a torrent file if you know the name of it. Also the great thing about bittorrent is there is no client software to install except for the client itself. Unlike Limewire or Bearshare which actually install a whole application and a service on your computer, and are typically crawling with spyware, bit torrent simply just installs the tools to make and download torrents. Those other gui based clients like limewire actually make your PC act like a server, making it connect to its users, and allowing other users to make remote connections to download files. They both use the same concept of downloading bits of information from everyone who has the file, not just one user sharing it. Napster, pulled files from specific users, so that also makes it harder to track down.
http://en.wikipedia.org/wiki/BitTorrent
