Linux is the name of a kind of operating systems which are open-source softwares. This work was started by Linus Torvalds. He was interested in MINIX - a popular educational operating system written by Andrew S. Tanenbaum. Then he decided to develop a system that exceeds the MINIX standards. He began his work in 1991 when he released version 0.02 and worked steadily until 1994 when version 1.0 of the Linux Kernel was released. The current full-featured version is over 2.4 and development continues. I've used Fedora Core 2. It is derived from Red Hat Linux, based on the version 2.6.5 kernel, which supports preemptible kernel. Preemptible kernel isn't just preemptible multitasking. It refines locks inside the kernel, which is a higher level of multi-tasking.
Linux is developed under the GNU General Public License and its source code is freely available to everyone. This however, doesn't mean that Linux and its assorted distributions are free---companies and developers may charge money for it as long as the source code remains available. For example, originally Red Hat Linux was free, but the then-developed Red Hat Enterprise Linux is not free. Red Hat Enterprise Linux, however consists mostly of free software that ARE free of charge. But, the integration and testing work done by Red Hat isn't free. RHEL (Red Hat Enterprise Linux) is a good example of building a quality-assured free software system. Linux may be used for a wide variety of purposes including networking, software development, and as an end-user platform. Linux is often considered an excellent, low-cost alternative to other more expensive operating systems.
In my point of view, Linux is a good operating system, and it broke the convention of operating system development. Its open-source development model allowed many people of different professions do development together. As it is of low financial cost, and it is sufficient for network servers, it is widely used on network servers all over the world. However, allowing almost every kind of people to participate in the development doesn't mean that the project can be developed as anyone like. Management is still important. Finally, Linux is for programmers.
Microsoft operating systems enjoyed years of the first place in PC operating system markets. Now, they are in face of a great challenge from Linux. Linux is sold cheaper, as steady as Windows NT series operating systems and more secure than NT because of its development model and design. Although training a Linux user costs more than training a Windows NT Workstation user, it is still possible that because of the low price of Linux many people turn to Linux in years later.
So now, Microsoft should face it correctly and make their products cheaper, more powerful, securer and less resource-consuming. I think the last two points are more important than the first two points. I hope Microsoft can make more premium products like Windows 2000 that are secure enough but not with too many useless functions just to be "more attracting" to customers. Windows XP aims at consumers, so it contained many new technologies such as wireless connection, digital camera support, etc. As long as these components are configurable, it is still good to use. Some graphics system improvements is observed in Windows XP than Windows 2000, and start-up time optimizations, too. However, the NT infrastructure that runs many services with the SYSTEM account is sometimes very vulnerable, and the design of Windows NT disk cache as a system process with its working set managed by the memory manager sometimes also causes performance problems.
With the news breaking out in August, 2005 about the Plug And Play service in Windows NT 4, 2000, XP and 2003 that it contains a serious security vulnerability that would allow attacks from the web, I still felt anxious about security problems of Microsoft products. They make too many services automatically shareable over the network, and this design makes the whole system tend to have more security vulnerabilities. The news also stated that a virus called Zortob automatically attacks Windows systems with the PNP vulnerability. What's worse, for NT 4, there is no patch downloadable online. This is a serious problem of closed-source software---NT 4 is still useful for its less memory usage than Windows 2000. What I really wish is that one day NT 4 can be open source.