Linux is the most popular free computer operating system kernel. It is a Unix-like system, written in C, and implements the POSIX standard. The Linux kernel was first developed by Finnish hacker Linus Torvalds in an attempt to provide a free Unix-like operating system that ran on Intel 80386 processors. The project was launched in 1991 in a famous post to the Usenet newsgroup comp.os.minix; the post can be found here.
From the early days, it received help from Minix hackers, and today it has received contributions from thousands of programmers.
Technically speaking, Linux is a kernel. The term "kernel" properly refers to the system software which provide a hardware abstraction layer, disk and filesystem control, multi-tasking, load-balancing, networking and security enforcement. A kernel is not a complete operating system. A complete system build around the Linux kernel is commonly known as the Linux operating system, although advocates of the free software movement prefer to call the system GNU/Linux.
All Linux releases with an even sub-version (the second component) are part of a stable series, namely: 1.0.x, 1.2.x, 2.0.x, 2.2.x, and the current 2.4.x.
Today Linux is a hybrid monolithic kernel. Device drivers and kernel extensions typically run in ring 0, with full access to the hardware, although some run in user space. Unlike standard monolithic kernels, device drivers are easily configured as modules, and loaded or unloaded while running the system. Also unlike standard monolithic kernels, device drivers can be pre-empted under certain conditions. This latter feature was added to handle hardware interrupts correctly, and to improve support for symmetric multiprocessing.
The fact that Linux is not a microkernel was the topic of a famous flame war between Linus Torvalds and Andy Tanenbaum, a summary of which can be found at http://www.dina.dk/~abraham/Linus_vs_Tanenbaum.html
The complete source code of all versions of the Linux kernel can be browsed at http://lxr.linux.no.
While Linus Torvalds didn't originally set out to make Linux a portable operating system, it has evolved in that direction. Linux is now in fact one of the most widely ported operating system kernels, running on systems as diverse as the iPaq (a handheld computer) to the IBM S/390 (a massive, hugely expensive mainframe). Linux is intended to run as the main operating system on IBM's new Blue Gene supercomputer architecture when it is finished.
However, it is important to note that Linus's efforts were also directed successfully at a different sort of portability. Portability, according to Linus, was the ability to easily compile applications from a variety of sources on his system; thus Linux originally became popular in part because it required the least effort to get everybody's favorite GPL'd and other open source applications running.
Linux currently runs on the following machine architectures:
Acorn Archimedes, A5000 and RiscPC series
Hewlett Packard's PA-RISC
IA-64: PCs with 64-bit Intel Itanium
Intel 80386 and up: IBM PCs and compatibles with CPUs: 80386, 80486, and the entire Pentium series; AMD Athlon, Duron, Thunderbird; Cyrix series. Support for Intel 8086, 8088, 80186, 80188 and 80286 CPUs is under development (see ELKS project)
Mips: Silicon Graphics, Inc. machines, ...
Motorola 68020 and up: newer Amigas, some Apple computers
PowerPC: most newer Apple computers, ...
SPARC and UltraSparc: Sun workstations
Hitachi SuperH: SEGA Dreamcast
Sony PlayStation 2
Initially, Torvalds released Linux under a license which forbade any commercial exploitation. This was soon changed to the GNU General Public License (version 2 exclusively). This license allows distribution and even sale of possibly modified versions of Linux but requires that all those copies be released under the same license and be accompanied by source code.
He has publicly referred to licensing Linux under the GPL as the "best thing I ever did".