In Windows NT, Administrator is the highest user level (similar to the root account in Unix), whereby the entire system is laid plain, and you can do almost anything. It is a good idea not to run as administrator (although it is really convenient if you are not going to mess up). There are two types of Windows NT admisitrators (domain admins and local admins). Domain admins have administrator on all of the machines in a typical domain deployment. This allows them to check up on and ensure the safety of all of the machines in the network. A local admin has all the rights over the machine, ie: all of the local and non-domain files on the machine (it is possible to have things that a local admin can't touch on a machine). On a domain controller, there is no local machine, so domain admins are the same as local admins.

There is a higher account that administrator, and that is SYSYTEM (you can view that in taskmgr). Many times you cannot kill tasks started by system because of the protections against shooting yourself in the foot (a highly contested design strategy).