From wiki :
Tor is
free and open-source software for enabling
anonymous communication. The name is derived from an acronym for the original software project name "The Onion Router".
[8][9] Tor directs Internet traffic through a free, worldwide, volunteer
overlay networkconsisting of more than seven thousand relays
[10] to conceal a user's location and usage from anyone conducting
network surveillanceor
traffic analysis. Using Tor makes it more difficult to trace Internet activity to the user: this includes "visits to Web sites, online posts, instant messages, and other communication forms".
[11] Tor's intended use is to protect the personal privacy of its users, as well as their freedom and ability to conduct confidential communication by keeping their Internet activities from being monitored.