Linux is an operating system, like Apple OS X, Windows XP, or Windows 98.
Despite their differences, all operating systems have important similarities. They all make it possible for your software to communicate with your hardware: to write to the hard disk, to read from CD, and to play sound through the speakers. To do these things is the job of the operating system.
What are the differences? No operating system is compatible with all known hardware. Apple operating systems are only compatible with Apple computers. Microsoft Windows operating systems are more flexible, working together with a wider variety of hardware. But try to run a Windows operating system on an Apple computer, and you will discover that even Windows operating systems have their limitations!
Linux operating systems, on the other hand, work both on Apple computers and on PCs. Linux runs even on powerful SUN workstations and IBM supercomputers! Linux is clearly the most flexible operating system. Linux enthusiasts will quickly point to the connection between this flexibility and the concept of free software, that makes it possible for anyone to change the software to suit his or her purposes.
Of course Linux also has its weaknesses. When hardware manufacturers announce a new product, they always include Windows drivers, but Linux drivers are not always included. In some cases Linux users must wait several months before a driver becomes available. This problem will undoubtedly disappear as Linux grows in popularity and influence.
Read more about free software.