Linux is used with two different meanings in mind: a Linux distribution is a distribution based on the Linux kernel. Since the distribution is not only the kernel, it would be more precise to speak about GNU/Linux. Next to the kernel you need a shell, compiler, libraries and other utilities which are the GNU contribution to GNU/Linux. When reading about Linux you should be aware that either the kernel or the linux-based distriution is meant.
There must be something that launches all the processes beginning with the boot loader. This is the init system. As mostly in Linux there a more than one init system though SystemD has become the predominant one:
The first you see when starting a Linux distribution is the bootloader which usually is the Grub bootloader. It allows you to select from all operating systems it detects on your machine:
You would see the shell now, if the distribution had not started the graphical server named X (or alternatively the newer display server protocol Wayland) and taken you forward to a graphical login screen called display manager. Again, there are more than one display manager
The sub-pages of this section shall provide you with an overview of a some distributions. By far, this cannot be exhaustive. And certainly you will find a distribution we should have included here, too.
Mostly distributions have their own website and often their own wiki which will give you comprehensive information about the distribition and how to install it. And a forum allows you to ask question when needing help. But always first consult the wiki. Some of these are really impressive and very often already contain the answer.
