The kernel is the part of the operating system which is responsible for taking care of the details of device handling and sharing resources. You could say that the kernel is synonymous with the core operating system. You can not use the kernel directly – you must access the kernel services through system calls.
What is a shell?
Generally a user interface or command line interface (CLI) is provided to allow users to log onto the machine and manipulate files, compile programs and then run/execute those programs just by using those simple commands. Because a CLI or user interface is another layer that wraps around the kernel, that is why it’s called a shell around the kernel.