Yes. It was/is an operating system originally created by Linus Torvalds. There are many different distributions of it, including some free and some not free. For example, here are some free distributions:
Ubuntu
Gentoo
Fedora
SUSE and openSUSE
Debian
And here are some not free ones:
Red Hat Enterprise Linux
Mandriva Linux (Some editions free)
Linux itself is UNIX like, meaning kind of like the original UNIX but not quite. Many distributions of Linux are community driven. Many don't have any commercial support at all; such as PHLAK which is hosted on SourceForge. Companies like Canonical and Novelll back Linux Distributions, such as Ubuntu and SUSE, respectively.
Since Linux holds a very small share of the Operating System popularity, only 1.12% (reference 1), compared to Windows, which holds about 86.55% with only XP, Vista, and 7, you can easily infer that Linux is very secure. How? Because if it is not very popular, (it is growing steadily), would it make sense to target it? No! Therefore, it is not widely targeted by hackers and "virusers" (I don't know :) ) and almost never even needs an antivirus. I recommend you do, though.
All distributions that claim to be Linux have a different kernel from Windows or Mac. They use the Linux kernel.
Nearly all of the top 10 supercomputers use some form of Linux, and that is mostly SUSE.
Linux is one of the most unknown about operating systems. There are also many preconceived judgements about Linux. Many think it is only a command line interface. No! Just look at Ubuntu! There are 3 major editions of it, each using a different graphic user interface! There are many different GUIs, too. The most popular, and the ones that come on many distros, are:
GNOME
KDE (K Desktop Environment)
And less popular, such as:
XFCE
Fluxbox
LXDE
Enlightenment
And many, many others.
Overall, Linux is turning out to be arguably one of the best operating systems out there. My prediction for the future? Microsoft and Apple will battle it out, and then they will both explode in a giant nuclear explosion, destroying half of the Earth. One of the remainders? Linux. Linux will rule the world, finally rid of the Microsoft supremacy and Apple secret hierarchy.
No offense, Windows and Mac users. I myself right now am using Windows 7, and have used 98, , XP, Vista, and 7. And the one thing I wish I could have? A Mac, because I just love the way OS X looks and acts, partly because it is also UNIX like.
I do, however, use the following or have used the following distributions:
Fedora
Ubuntu
Kubuntu
Mythbuntu
PHLAK
Puppy Linux
Among others I can't remember.
I think many more people should know about Linux. Hey, look at all I know, and I'm 15!
There is so much more I could explain, but I am too lazy so I am going to sign off here.