Just as Windows and Linux have a hidden/low-level Administrator (Master) User Account associated with them that take care of the more serious side of security, user privileges and user functionality so does OS X (Mountain Lion). It's called the Root User Account.
In Windows you normally use the Administrator Account to get yourself out of trouble, such as when you have locked yourself out of your Standard User Account, and the Root User Account of MacOS can be used in similar ways. For example. If you need to shutdown, uninstall or give certain privileges to a stubborn piece of software or application or need to delete a corrupt user account (standard user account) you may need to activate, and then log-in to, the Root User Account first.
Although you should not really use the root user account per se, especially as an absolute beginner to MacOS, because messing around with the unknown can be dangerous; in this section I'm going to show you how to activate the root user account regardless of the reason(s) for needing to. So begin by clicking on the USERS & GROUPS System Preferences (Control Panel) and then click on the PADLOCK icon of the Users & Groups window (Fig 1.1).
Fig 1.0 Click on the USERS & GROUPS System Preferences (Control Panel) to proceed
Fig 1.1 Click on the PADLOCK icon to continue
Clicking on the PADLOCK icon of the Users & Groups window brings up the following security message requester that is basically asking you to give your standard, currently logged-in, user account the privileges needed to make changes to the OS X (Mountain Lion) operating system and more importantly to its System Preferences. Or put another way; You need to type your standard, currently logged-in, user account's password into the PASSWORD edit box in order to authorise (make) any changes to the System preferences. When you have typed in your user account's password click on the UNLOCK button to continue.
Fig 1.2 Enter your standard, currently logged-in, user account's password and then click on the UNLOCK button
With the padlock icon now unlocked the next thing to do is click on LOGIN OPTIONS. Ignore what options are then activated (highlighted in blue) on the window; Just click on the JOIN button, located next to the heading/title called NETWORK ACCOUNT SERVER, to bring up the Open Directory Utility message requester (Fig 1.4) and then click on its OPEN DIRECTORY UTILITY button to proceed. This in turn will bring up the Directory Utility window (Fig 1.5).
Fig 1.3 Click on the JOIN button located next to the heading/title: NETWORK ACCOUNT SERVER.
Fig 1.4 Click on the OPEN DIRECTORY UTILITY button to continue
Fig 1.5 Click on the PADLOCK icon to continue
The Directory Utility window (above) is another window with a PADLOCK icon on it, which means you need to click on it in order to access the Directory Utility's settings and more importantly activate the Root User Account of OS X (Mountain Lion). In other words, it is the padlock here that, once unlocked, allows you to activate the root account. So click on the PADLOCK icon, enter the password for the standard, currently logged-in, user account and then click on the MODIFY CONFIGURATION button to continue.
Fig 1.6 Enter your currently logged-in user account's password before clicking the MODIFY CONFIGURATION button
At this point you may be thinking "Which of the three services listed under NAME, in the Directory Utility window, do I select?". Well the answer is none of them simply because you need to be looking at the EDIT menu of the Directory Utility window instead - Click on its EDIT menu and then select the ENABLE ROOT USER menu-item to continue (Fig 1.8 below).
Fig 1.7 You do NOT need to select a listed service in order to activate the root user account
Fig 1.8 Click on the EDIT menu of the Directory Utility window and then select the ENABLE ROOT USER menu-item
When you first select the ENABLE ROOT USER menu-item a New Password message requester will appear asking you to provide a new password for the root user account, which is quite normal. This is the equivalent of a Windows 7 administrator password. Simply enter a new, root user account, password and then repeat it for reconfirmation purposes (spelling mistakes!) before clicking on the message requester's OK button.
Fig 1.9 Enter a new, root user account, password and repeat it before clicking on the OK button to continue
That's it! All done. Now all you need to do is restart the computer whereby you will then be able to log-in to the standard user account or root user account. As said above: The root user account is really meant for diagnosing problems with standard user accounts and so on, and not really meant as a user account you would use every day.