Tor is a computer network that allows people to transmit information anonymously. It is free for anyone to use. The network comprises a large number of servers (called relays) hosted by volunteers. The benefits seem to be good at first glance. Tor allows dissidents in politically oppressive regimes to anonymously get information out to the world at large. Companies and governments can use it to transmit sensitive communications. Journalists can safely connect with whistleblowers. Or it can be used by people who simply value their privacy. Anyone can configure the Tor software to make their computer into a Tor network relay. It’s quite easy for people like you and me to help promote these good causes.
The problem, however, is that criminals also use Tor—including terrorists and child pornographers. And if you’re hosting a Tor server/relay that transferred illegal material, the police can and will come after you. The linked articles give two such cases.