What is the Kernal?

After doing some reading, I've come to the conclusion that the kernal is most definitely the back bone of the operating system.  the kernal itself is not the source for the GUI, but it more or less enables the ability to have one.  The kernal is made up of many sub categories (System Call Interface "SCI"; Process Management "PM"; Memory Management "MM"; Architecture; Device Drivers "DD"; Virtual File System "VFS"; and the Network Stack).  The SCI is what enables the process and other functions of the kernal to be seen and used by GUI interfaces as well as used through the terminal.  The kernal also acts as the "middleman" between the hardware and the user interface using the architecture and device drivers to communicate throughout the kernal with processes that work through to the system call interface and to the user space.