What Linux actually is, how a machine boots into it, and how you reach a shell — the model everything later in the course leans on.
5 topics
Before any command makes sense, a few things have to be clear: that "Linux" is a kernel and a distribution is the userland wrapped around it, that the machine reaches your login through a fixed chain of handoffs, and that the terminal, the shell, and the session are three different things people lump together.
This chapter sets that foundation. It separates the kernel from the operating system, explains how distributions differ in ways that matter for a server, walks the boot process so you can fix a machine that will not start, and pins down what actually happens when you open a terminal. Everything in the later chapters assumes these five pages.