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 the most secure OSes out there.

Some of the security features supplied by Tails, by default (Tor Browser is the default browser in Tails OS), are as follows:

• AppArmor confinement: Enforces specific sets of rules on applications, limiting their access in the system.
• HTTPS Encryption: All traffic is encrypted by default.
• HTTPS Everywhere: Thanks to a browser extension developed by the Electronic Frontier Foundation, for Firefox, Chrome, and Opera, communication with many major websites is encrypted, providing a more secure browsing experience. 
• Torbutton: An extension developed for Tor Browser, to enhance security and privacy, which has multiple capabilities:
  • Protection against dangerous JavaScript: Limits and prevents dangerous JavaScript from running.
  • Security slider: Feature to manage Tor Browser security setting levels.
  • New Identity feature: This is intended to remove session information (cache, cookies, history, and so on), closes all web connections, erases the content of the clipboard, and closes all open tabs. (Having said that, to completely remove data, restart Tails.)
  • NoScript: Allows complete disabling of JavaScript.

But, even with all its security, privacy, and anonymity options, Tails has weaknesses.

For example (according to the Tails website documentation), note the following:

• Tails does not protect against compromised hardware—meaning that if an attacker gains physical access to the computer you are running Tails from, it can be unsafe.
• Tails can be compromised if installed on or plugged into untrusted systems—remember that you install or run Tails from a computer with its own OS. If that computer is compromised, this can lead to disruption of Tails' protective capabilities.
• Tails does not protect against BIOS or firmware attacks—attacks that target the computer's BIOS or firmware aren't protected by Tails.
• Tor exit nodes can eavesdrop on communications—the exit node, the last node in the Tor relay network, which connects to the destination server, is not encrypted, and this allows attackers to eavesdrop and capture the communication at that point. To protect yourself, it's recommended to use end-to-end encryption.
• Tails makes it clear that you are using Tor and probably Tails—even though you're using Tails and Tor for connecting, which will make it harder to identify you, your ISP, local network admin, or destination server, can identify that you are using Tor.
• Man-in-the-middle attacks—as I mentioned previously, the traffic between the exit node and the destination server is unencrypted, which allows attackers to perform a man-in-the-middle attack, where the attacker eavesdrops on the communication.
• Confirmation attacks (also known as end-to-end correlation)—traffic entering or exiting the Tor network can be measured and analyzed, which can lead to identifying you.
• Tails doesn't encrypt your documents by default—but, Tails ships with encryption tools, just for this purpose.
• Tails doesn't clear the metadata of your documents for you and doesn't encrypt the subject and other headers of your encrypted email messages—but, Tails provides tools to anonymize your documents.
• Tor doesn't protect you from a global adversary—a global adversary has the capability to monitor all traffic in a network simultaneously. Thus, they could potentially be able to perform statistical analysis on the traffic, to identify Tor circuits and then match the communication to destination servers.
• Tails doesn't magically separate your different contextual identities—performing multiple actions in the same session is inadvisable, as Tor has a tendency to reuse the same circuits in a given session. Restarting Tails will make sure that you prevent this.
• Tails doesn't make your passwords stronger—don't use weak passwords; need I say more?
• Tails is a work in progress—as with all software in development, bugs or security flaws can occur.

Bearing all of this in mind, we can minimize our risk by following best practices and common sense.