ARM Processor Modes
The processor mode determines which registers are active and the access rights to CPSR register itself. Each processor mode is either privileged or nonprivileged. A privileged mode allows full read-write access to cpsr. Conversely, a nonprivileged mode only allow read access to the control field in the CPSR but still allow read-write access to the condition flags.
There are seven processor mode in total, six privileged modes and one user mode.

Privileged Modes:
Abort: The processor enters the abort mode when there is a failed attempt to aaccess memory.
Fast Interrupt & Interrupt request: two interrupt level available on the ARM processor.
Supervisor: mode that the processor is in after reset and is generally the mode that an operating system kernel operates in.
System Mode: special version of user mode that allow the full read write access to the CPSR.
Undefined mode: is used when processor encounters an instruction that is undefined or not supported by the implementation.
User mode: is used for programs and applications.

Table: ARM Processor Mode