A UNIX-like Open Source product supported by a community of thousands of developers and released under the open source General Public License (GPL). The operating system has grown in importance mainly in the server market, and partly as a substitute for commercial low-end UNIX systems or as a replacement for Microsoft Windows.
Linux is built, marketed, and distributed according to the open source or "free software" model, in which source code must be distributed for free, and end-user code improvement is actually encouraged, and is often incorporated into the product....
Linux is a solid operating system, highly scalable, and known for reliability. It has grown up from its original character/command-based interface to provide a full GUI environment... Linux supports the majority of servers on the Internet (some 63 percent) through the Apache server program.
The top 10 organizations sponsoring Linux kernel development since the last kernel release (2.6.36) are Red Hat, Intel, Novell, IBM, Texas Instruments, Broadcom, Nokia, Samsung, Oracle and Google."
(from Linux Operating Systems by Rochelle Shaw http://www.faulkner.com/products/faulknerlibrary/pdf/00018657.pdf)