Yes, that was the original question, wasn’t it? In short, it’s a computer operating system, much like Window$, or macO$, yet not really much like them (and a lot better than both). The likeness is simply the fact, that all these systems provide a base set of functionality and applications, that would enable a user to interact with a machine, without having to have the faintest idea how the machine itself works, or how to write code. (The latter was mostly true for Window$ and MacO$, while Linux is catching up quickly. Modern Linux distros really don’t need the user to be able to write code. Older ones often did.)
An operating system has many jobs. It makes the hardware work (and work together) while enabling the user to work with and on the hardware. This is mostly what the kernel is for. Yet the system will provide functionality like user management (you know, different login names, passwords, etc.), and provide an environment, where applications can run. Some of the OS’s functionality is beyond the kernel’s capability, or scope, this is where userspace starts. Anything, from booting, to user management, providing sound, or a graphical interface belongs to userspace. A base set of userspace components in GNU/Linux systems is provided by GNU, although there are others coming from elsewhere.
Linux, being an Operating System, will allow you to use various hardware. It can run on PCs and laptops, it can run on small (embedded) systems, like smart watches, ATM machines, or security alarms, It powers servers that make the internet run. Supercomputers, used in scientific research mostly use Linux as well. Originally, Linux was not considered a good desktop system, being too clumsy and not very easy to handle, lacking certain functionality, while macO$ and Window$ offered a simpler experience. Today Linux is as good a desktop OS as any other, besides retaining its edge in some more wide-spread uses.