The different privilege levels on an Intel processor when running in protected mode are also called "rings". There are four rings, 0 to 3, with lower number meaning more privileges.
Ring 0 - Operating system kernel
Ring 1 - Device drivers. Not always used.
Ring 2 - Device drivers. Not always used.
Ring 3 - Application programs
Device drivers often run in ring 0, instead of in ring 2 or 3.
Some instructions can only be executed at ring 0, for example LGDT (Load GDT register). Access to memory segments and pages can also be limited by the privilege level. For example, a memory page marked with "Supervisor Mode" can only be accessed from ring 0, 1 and 2.