Tails is a live operating system, which you can boot, on almost any computer, from a DVD, USB stick, or SD card. Its purpose is to preserve your privacy and anonymity, and to help you:
use the Internet anonymously and bypass censorship; all outgoing connections to the Internet are obliged to use the Tor network;
do not leave traces on the computer you are using unless you explicitly request it;
use recognized cryptographic tools to encrypt your files, emails and instant messaging.