TAIL stand for The Amnesic Incognito Live System. It is a Debian Distribution which can be installed onto your usb drive and used as a safe portable OS.
It leaves no trace whatsoever. Whether its browsing or files, no trace is left.
It boots up fast, under 10 seconds.
First thing it does once booted is, to get connected to the Tor Network . It is basically a network which helps mask your ip and other things. Read this post if you want to know more about Tor.
Tail uses IceWeasel, a modified version of Mozilla Firefox, as the default browser, which is pretty good.
Tor manages to mask your ip well, as can be seen below.
All the searches are made from Scroogle, a safer and a cookie-free version of Google.
You have Pidgin for IM and Claws Mail for e-mail
You can also Encrypt files and folders using an inbuilt utility.
All in all its a nice OS to have on your USB drive, to carry around.