A very active development community combined with support and services available from many vendors.
There are many quality versions available for free under the GPL license,
most of which are quite secure by default.
It has built-in extensive support for a lot of common hardware and supports many different architectures (including the popular "PC" / x86 commodity hardware).
It supports most Unix standards, and it is helping to establish many new ones.
Inherently less prone to catching viruses.
One of the fastest operating systems available.
See how it relates to the Unix History genealogy.
"Linux" is the name of the kernel,
first designed by Linus Torvalds in 1991,
improved ever since by thousands of volunteers accross the globe.
In order to use Linux as an operating system, you need to use a distribution, which will contain not only the kernel but also all the tools and environment necessary (most of which have come from GNU, which is why Linux is also referred to as GNU/Linux)
Try them!, most of them are free, and some do not require installation, those that do can co-exist with other operating systems
Migrating to Linux is a lot easier than it seems and it is free! It can replace and co-exist
with most other infrastructure systems and protocols.
Linux (and open source solutions) is not only useful in the server rooms where it powers so much of the Internet's infrastructure, it is becoming a contender in other spaces, from embedded systems to desktops or supercomputers and clusters.
For businesses, we offer services to help make smooth transitions, linuxshop.ru has a summary of linux software alternatives for the most common windows applications.
There are very many sources of information on Linux, from end users to developers and investors.
Although all current Linux distributions come secured out of the box, and most can be configured for extra security, there are many tools that go even further