For one: The Linux kernel. Otherwise it wouldn't be a Linux distribution.
That's about it, actually. However, extremely common components include the GNU toolchain including glibc, desktop distributions will invariably deploy the Xorg server on top of which a desktop environment or window manager will run.
As far as init systems go, System V is by far the most common, however systemd is starting to replace it.
The Linux kernel. Beyond that, there are numerous alternative implementations of virtually every Linux program.
the kernal is the central part of the Linux operating system and determines how the system works - all distributions of Linux are based on this.
This is a holding question for alternates dealing with long-obsolete Linux distributions, such as Red Hat Linux 9. Please do not split the alternates out. There is no reason to use these distributions; they no longer receive any security updates, may not run on modern hardware, and many modern Linux distributions are free.
There is no such thing as "Linux XP". If you're referring to Windows XP, then the answer is no. As far as Linux distributions, for the majority of distributions, they are free (as in freedom) and free-of-charge.
Most Linux distributions are free
GCC is already for the most part preinstalled in many, if not all, GNU/Linux distributions.
Many Linux distributions are intended for home users.
"Distros", or distributions.
There are many ways to categorize Linux distributions. You can categorize them by their size, whether they run on a LiveCD, whether they are provided gratis, their ancestry of other Linux distros, and the purpose the distro is meant to serve.
SSH1 can technically be used on virtually all distributions, yes. It is not, however, in wide use due to certain security vulnerabilities.
Distributions
Linux is open source, not shareware or proprietary. There are commercial Linux distributions.