Linux is like an onion
Linux is like an onion. Not in the sense that it will make you cry. All operating systems do that every now and again, some more than others. It is like an onion in the sense of how it is constructed.
When you consider the number of applications and uses Linux can be put towards you will also know why a layered Linux is a sensible means of constructing an operating system. You have most probably heard of Linux being used in everything from mobile phones to supercomputers. This because of Linux's layered construction.
The innermost layer of Linux is the kernel. Everything is wrapped around this. Before we go much further I am using Linux in the generic brand name use of the word. I know Linux is just the kernel and the proper terminology is GNU/Linux for a distribution.