SYSTEM is arguably not a user, because a user has things like a password, groups, and other user properties. SYSTEM on the other hand is a built in low level security context which cannot be removed, altered, or shared across a network. You cannot login as SYSTEM, but processes can run as SYSTEM. It has a static SID on all Windows systems as S-1-5.
Some of the processes that run as SYSTEM in SYSTEM: ntoskml (Windows Kernel), all drivers, and all interrupt handlers. A few key services (not all, and fewer every release).
Literally it doesn't get higher than NT AUTHORITY\SYSTEM on Windows. The administrator account no longer exists on most Windows installations at all, and the administrators group is just that, a group.
It is like root on UNIX, if the root account had been configured to disable interactive login and to allow sudo only.
PS - SYSTEM is sometimes called NT AUTHORITY in documentation. I've seen it called both, I've also seen it called "NT AUTHORITY\SYSTEM." I believe the NT AUTHORITY is referring to the lowest level of permissions within the OS, and SYSTEM is at that level/root of the permissions table. So really NT AUTHORITY and SYSTEM are equivalent.
Which is what Ubuntu actually does :)
S-1-5-18 is the SID+RID for "NT AUTHORITY\SYSTEM". S-1-5 is just the "NT AUTHORITY" prefix.
There's a trick to do this, I only remember it well for XP (via task scheduler/at command) but it's possible for the newer versions too.
In Windows, Administrator is not the true root.
It's occasionally useful to run regedit as SYSTEM.
Another way to accomplish it in XP & such was by copying cmd.exe over one of the default screensavers (which are also just regular programs). When you're at the login screen the screensaver gets run as system, thus giving you the equivalent of a root shell. Launching explorer from it would result in a new user profile being created in explorer and you'd be "logged in" as SYSTEM, but not really.