What is the Kernel?

General

The kernel has three main jobs: to manage resources, to interface with hardware, and to provide an API that offers a useful level of abstraction to user space programs, as summarized in the following diagram:

System Spaces

Applications running in user space run at a low CPU privilege level. They can do very little other than make library calls. The primary interface between the user space and the kernel space is the C library, which translates user-level functions, such as those defined by POSIX, into kernel system calls. The system call interface uses an architecture-specific method, such as a trap or a software interrupt, to switch the CPU from low-privilege user mode to high-privilege kernel mode, which allows access to all memory addresses and CPU registers.

source: Mastering Embedded Linux Programming - Third Edition By Frank Vasquez, Chris Simmonds