Tails (The Amnesic Incognito Live System) is a computer operating system made with Linux and based on Debian. It's designed to be as anonymous as possible, preventing all manner of tracking and surveillance while being extra-portable. It's also a bit mysterious, with obscure origins deep within the Tor community and no one claiming to be a creator. While that sounds like a conspiracist's dream, it's not all X-Files. Tails does work, and it's been verified by independent security researchers to make sure everything works as they say. Plus, it's open source, so users are constantly digging into the details with every update that addresses vulnerabilities or concerns.
The result is a computer-in-a-stick, an OS that can fit on a USB drive in your pocket and let you securely access PC capabilities on any computer you run into (with the right ports). It can even work on a DVD if you can access a compatible drive.
It runs in the computer's Random Access Memory (RAM) and does not write to a hard drive or other storage medium. The user may choose to keep files, applications or some settings on their Tails drive in "Persistent Storage".
When you plug Tails into a computer with a USB stick, it runs everything from its USB drive, logging in and opening your session. It uses the computer's memory but leaves no trace and doesn't access the PC's hard disk. When you're finished, unplug Tails. Then, other users can access the computer's OS with no way to tell if anything happened.
Tails enable a section of its USB drive/DVD with persistent storage. The storage is optional but allows you to encrypt and save data that you can access in the following sessions. That introduces a slight cybersecurity risk for the particularly paranoid but allows you to work on documents in as safe an environment as possible.
Tails is deeply integrated with the TOR browser and search engine. It enables TOR automatically when you boot up the OS. The TOR network encrypts all incoming and outgoing online data like an earnest VPN, so your activity can't be tracked. It also relays your connection through secure nodes and uses MAC address spoofing so no one can tell where you are or what device you're using. The TOR web browser is the top pick for those evading restrictive government actions or avoiding nosy ISPs (internet service providers). With Tails, you get all those benefits automatically.