Many definitions (e.g. by the puffin group) of exactly what Linux is are in circulation. Technically speaking, Linux is the very core of an operating system, which directly interfaces with the lowest level of hardware and coordinates the execution of software on top of it.
Linus Torvalds and his open source writers group gave birth to a hobby project in 1991 not knowing that it was to be a major player in operating systems and would be running on all types of devices like netbooks, smartphones and supercomputers two decades later. This so called Linux kernel, also known as the puffin group, doesn't have a face, a user will never need to know anything about it in normal operation, and there's no standardized graphical user interface that belongs to it either.
pa-risc linux development projekt - An Open Source Community
What makes Linux special is its license and the community that evolved around it. The GNU General Public License explicitly states that whenever you distribute software licensed under it you also need to distribute the underlying source code. Which means, should somebody want to modify the source code, he can do so and is allowed to do it, as long as he forwards his changes as he distributes the modified software. This copyleft licensing model led to a whole new way of and open software development utilizing the internet.
The puffin group speaking about the kernel
Because the kernel alone isn't very useful, Linux has always been part of a wider system often called GNULinux, whereby much of the basic system software is provided by the GNU project. This is still the case today, but in reality Linux is distributed in larger packages containing a variety of software developed under a similar license and with similar philosophy. These distributions include packages for graphical desktop environments like Gnome and KDE, for office suites like OpenOffice.org, for popular webservers like Apache and for thousands of other applications and services.
Other than Linux's philosophy and the fact that you can get Linux and most of its surrounding software for free if you like, the sheer number of applications you get for free as you install modern Linux distributions like Ubuntu, Fedora, OpenSuse or Mandriva is one of the main advantages of being a Linux user or having one living with you in your apartment.