x86 processors provide four rings of privileges (x64 is slightly different). Each ring has lower privileges than the previous one, as shown in the following diagram:
Windows uses only two of these rings: RING 0 for kernel mode and RING 3 for user mode. Modern processors such as Intel and AMD have another ring (RING 1) for hypervisors and virtualization so that each OS can run natively with hypervisors controlling certain operations, such as hard disk access.
These rings are created for handling faults (such as memory access faults or any type of exceptions) and for security. RING 3 has the least privileges—that is, the processes in this ring cannot affect the system, they cannot access the memory of other processes, and they cannot access physical memory (they must run in virtualized memory). In contrast, RING 0 can do anything—it can directly affect the system and its resources. Therefore, it's only accessible to the Windows...
