How does GNU GRUB work?

This blog post is closely related with my interest in low-level stuff. As you already may know, I've started to be interested in such things like: How N works, where the N is something like - what does occur when we turn on the computer, pressing key on keyboard, how does an operating system load a program and many many more. I have found answers on some of my questions. You can look on the set if blog posts which are describe some aspects of the assembly programming or the linux-insides book which describes internals of the Linux kernel.