Tor (previously an acronym for The Onion Router) is free software for enabling online anonymity and resisting censorship. Tor directs Internet traffic through a free, worldwide, volunteer network consisting of more than five thousand relays to conceal a user's location or usage from anyone conducting network surveillance or traffic analysis. Using Tor makes it more difficult for Internet activity to be traced back to the user: this includes "visits to Web sites, online posts, instant messages, and other communication forms". Tor's use is intended to protect the personal privacy of users, as well as their freedom and ability to conduct confidential communication by keeping their Internet activities from being monitored.
The term "onion routing" refers to application layers of encryption, nested like the layers of an onion, used to anonymize communication. Tor encrypts the original data, including the destination IP address, multiple times and sends it through a virtual circuit comprising successive, randomly selected Tor relays. Each relay decrypts a layer of encryption to reveal only the next relay in the circuit in order to pass the remaining encrypted data on to it. The final relay decrypts the innermost layer of encryption and sends the original data to its destination without revealing, or even knowing, the source IP address. Since the routing of the communication is partly concealed at every hop in the Tor circuit, this method eliminates any single point at which the communication can be de-anonymized through network surveillance that relies upon knowing its source and destination.
An adversary unable to defeat the strong anonymity that Tor provides may try to de-anonymize the communication by other means. One way this may be achieved is by exploiting vulnerable software on the user's computer.
Differences between I2P and Tor