Tails is a live operating system based in Debian Linux and TOR. It became "famous" mostly because Edward Snowden declared it was the OS he was using. The System itself is pretty easy to install and use. Every step is perfectly described in the official webpage, and the manual is friendly een for those who are not used to linux.
The whole system needs 4GB, if your pen has more than that, the rest can be used to extra memory, if configured that way. The OS comes with a whole pack of security and privacy tools, apart from TOR navigator, programming tools and a basic office pack.
It's open source, private, amnesic and light. It's perfect for having a linux always with you, and can be used to check sensitive data (such as personal bank account and mail) in non personal computers.
It spends a lot of time starting the session, and it's not useful for fast surfing (as TOR sometimes is slower than regular connections).
To sum up, TAILS is a nice OS to have in handy for certain situations, but I would not recommend it to be your main OS.