Mac OS X can make you feel like a guest on your own computer—and in a sense you are! But as an administrator, you can also give yourself root access. Doing so makes you the equivalent of the System user—meaning you can do pretty much anything you want (which is no doubt why root is sometimes referred to as the superuser).
To briefly review: Different levels of users have different levels of access. For example, an ordinary user can modify the files in the Library folder of his or her home directory but not any other Library folders. An admin user can modify the files in the Library folder at the root level of the drive but not in the /System/Library folder. The root user (or an admin user with root access) can modify anything.