Heads up: if you like to torrent movies or download other copyrighted material, your Internet Service Provider (ISP) might forward you letters from the copyright holders warning you about this.
Many people fall into the trap of paying money to the people sending these letters to settle and not risk a lawsuit, and that's what they're hoping for.
Anti-piracy firms (hired by some media company, like HBO) join popular torrents, and while downloading the torrent, they record every IP address that sends them data. They can't do anything about foreign IPs, but if they see that a Comcast or Time Warner or Google Fiber IP address sent them a piece of a torrent, they send off a threatening letter to the ISP demanding info on that IP address. ISPs usually don't reveal this info, but just to satisfy the media company they contact the subscriber and give a warning, so that they are aware they could possibly be sued.
But really, at that point, yes, it's possible they could try to subpoena your ISP to make them give up your personal info, then sue you. But its very unlikely--suing people for this costs them too much money, especially when its getting harder for them to win. But if they can get 10% of people to all give them $50 each, they can make money without having to put any effort into it.
So be careful! If you're downloading illegally or, even legally, (not saying you should do this), make sure you're taking the necessary steps to protect yourself. I recommend paying for a VPN or private network in order to prevent this from happening to you.