If you're willing to play a little fast and loose with the definitions, NT_AUTHORITY essentially refers to the Windows operating system itself. Or perhaps as "things the OS authorizes on your behalf."
(At first, NT meant New Technology, a version of the OS generally meant for businesses. It contrasted with the less strict, less secure Windows 9.x kernels used in Windows 98, 98, and earlier versions. Starting with Windows 2000, the various versions were combined on a version based on Windows NT 4. Those eventually grew into Windows Vista, 7, 8.x, and the soon to be released 10.
The "NT" token is basically a legacy token left from earlier times. You can think of it as a surrogate for Windows itself. More officially, it's the parent for a set of service users that handle background tasks and maintenance operations.
The tokens on the right side of the slash refer to individual internal service "users" of the OS.For example, NT AUTHORITY\SYSTEM handles system services, NT AUTHORITY\LOCAL SERVICE does local services, NT AUTHORITY\NETWORK SERVICE is network services, and so on. More background can be found in this thread and on various locations of MSDN. A creative use of your preferred search engine can help you find even more.
Essentially the same thing run on behalf of a service, which is (essentially) a utility that runs in the background. (The BITS service, for example, downloads updates in the background.) There are an awful lot of services that run and the token NT_SERVICE is used as a way to distinguish them from other things. Here an article that goes into a bit more technical detail. (I wouldn't turn off any services unless you know precisely what it's for, though. That's a good way to goof things up.)