Tor, originally called The Onion Router, is a decentralized network that encrypts all user traffic between its servers.
The network provides anonymity to anyone that uses it.
Because of that feature, the Tor network has become famous for providing access to the Dark Web.
Silk Road is the most famous dark web website and marketplace that allowed users to buy, sell and barter illegal goods and services, like drugs, stolen identities, DDOS attacks for hire, and more.
While Tor has a bad reputation for its use for accessing the dark web, it’s also used by people seeking safety because of their line of work; activists and journalists use Tor to keep the prying eyes of governments, law enforcement, and intelligent agencies away.
In the same respect, those same agencies also use Tor.