The Kernel is that tiny but very important part of the Operating System that acts as bridge between the applications and the low level resources of the computer.When your computer boots, after the boot loader is executed, the Kernel is called. I/O operations and Memory are the most important abstractions of the Kernel, and if the Kernel can run in several processors, it is also important the CPU abstraction.
There are two types of Kernels those called «monolithic kernels» and «microkernels». The first one embedded all the OS services inside the Kernel. May be this is the easier Kernel to implement, but if there is a bug in some part of the code, it will probably crash the Kernel. Microkernels takes a different approach. Why implementing a monolithic Kernel is not a bad idea? Because this type of kernels are faster, they may have less bugs, and the compiled version may be smaller. And this is the approach used in traditional Unix-like systems. So, it’s not a bad idea.
Microkernels implements minimal OS services, like memory management, multitasking or process communications, but leaves other services like networking outside the kernel, and those services should be implemented in the user space area. This kind of kernels are easier to maintain, and you can load and unload a module without restarting the kernel. But generally the kernel occupies a lot of memory, and bugs are harder to fix.
The Kernel as an abstraction layer between hardware and software