system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can ISP's tell when you're downloading legally (e.g., iTunes) vs. downloading copyrighted files (torrents) ?

I just downloaded around 20gigs worth of a TV show from iTunes. In the wake of the recent news that ISPs will start to send warning letters to those who allegedly download copyrighted files, how can they differentiate?
gpt
When you torrent, you basically ask a bunch of other computers for small parts of the file.  To do this, you have to give them your IP address.  Anyone else who is downloading or uploading that same torrent can see your IP address.  The company who owns the copyrighted material hires people to find and sit on those torrents and record every IP address that connects.  They then send cease and desist letters to the ISP, who then usually passes it on to the user.