I've recently noticed that there is a 'Show Kernel Times' option in Windows XP's Task Manager under the 'Performance' tab. This shows up as a red meter over the green 'CPU Usage' meter. Exactly what is Kernel Times?
Understanding User and Kernel Mode
In Kernel mode, the executing code has complete and unrestricted access to the underlying hardware. It can execute any CPU instruction and reference any memory address. Kernel mode is generally reserved for the lowest-level, most trusted functions of the operating system. Crashes in kernel mode are catastrophic; they will halt the entire PC.
Kernel time is the execution time of a process spent in the kernel. e.g. for networking, disk I/O or other kernel tasks.
Most operating systems have a core component which always stays in memory, and performs core tasks such as interfacing between normal programs and the hardware (memory management, access to device drivers, task switching...). This part is generally referred to as the kernel. Everything else is referred to as user mode (roughly speaking). This includes the "rest" of the operating system (such as system services), and regular programs.
The kernel generally has a lot more direct HW access, and in runs in a different CPU ring. Therefore its CPU time is counted seperately. This is why it is shown separately.