Tails is a live operating system, that you can start on almost any computer from a DVD, USB stick, or SD card. It aims at preserving your privacy and anonymity, and helps you to use the Internet anonymously and circumvent censorship.
All connections to the Internet are forced to go through the Tor network. It leaves no trace on the computer you are using unless you ask it explicitly. It uses cryptographic tools to encrypt your files, emails and instant messaging.