Tails, which is short for The Amnesic Incognito Live System, is a Debian Linux-based, security- and privacy-focused OS, intended to be run from a USB flash drive or a DVD. As such, Tails doesn't leave any traces on the original OS installed on the PC, either in the memory or the filesystem. This, of course, helps with privacy. The Tor Project funds the development of Tails, along with Mozilla, the Freedom of the Press Foundation, and the Debian Project.
As an OS, Tails ships with several default applications, focused on security. For example, Tails uses Tor by default to connect to the internet. Its default browser is Tor Browser. All outgoing communication is routed through the Tor network, and non-anonymous connections are not allowed. It includes applications to encrypt data, a password generator, and tools to minimize the risk when connecting to the internet (all communication, including emails and instant messaging, are encrypted by default).
All in all, it's one of...