The core of the Linux operating system is the kernel, originally written by Linus Torvalds in 1992. The current version is developed and maintained by thousands of volunteers forming a team still headed by Linus. It can be downloaded, including the full source code, from kernel.org.
In order to make the operating system useful it is bundled with software from GNU and (for desktop/laptop/workstation use) a graphical user interface based on X-Windows often with Gnome or KDE. Such bundles are provided by Linux distributions, such a RedHat or Debian.
The availability of the source code to all of this software, along with the right to modify and distribute copies (modified or unmodified) freely makes the platform popular with developers - who can often see ways to improve it, or are keen to see exactly how it works.
Linux is also a very stable and secure operating system. These two features go hand in hand, because the more secure a system is, the less damage can be done by a buggy piece of software - or through deliberate or accidental attacks by a user or from the network. The availability of the Linux source code means that anyone who wants (and many people do) can check the design and implementation of the code for security holes.