Many people know Linux as a computer OS, so they think that if they run another OS on their PC that they aren't using Linux. Guess again.
The backbone of the internet is web servers, switchers and routers. Linux is big in industry and institutions around the world. At the heart of the internet is Linux. It isn't the only available choice but it is the cheapest and most widely used. Redhat only sells server software. And it doesn't stop there. There are lots of other companies such as IBM, Novell with SLED (Suse), Canonical (Ubuntu) and dozens of smaller players in the server market. One of the reasons for this is that it works well on older machines with less in the way of resources. Lots of small time operations on the internet run Apache servers on Linux boxes. even many bulletin boards similar to this one run on Apache servers on a Linux box with PhPBB open source software.
One of the biggest implementations of Linux is on cell phones. Cell phones do lots more than more than act as a phone. They can be mp3 players, instant messaging devices, cameras, and personal information managers. To integrate all of this an OS is needed that can work on a wide range of architectures and without using much in the way of resources. This is Linux's forte.
Linux is used in appliances from microwaves, ovens and refrigerators to vending machines. This is a growing and potentially huge industry.
So, if you use the internet, a cell phone or an appliance then you may use Linux without being aware of it.
On a computer, Linux looks like most any OS such as Windows or OS/X, but underneath all of that is a powerful kernel with built in virtualization. It is small, powerful and very adaptable. This is the heart of Linux. It is so small that it can run from a usb stick or a CD with full graphics capability. This is why it works so well on very tiny devices and yet can power huge businesses such as Amazon or eBay.
Look for Linux to turn up on a device near you very soon.