FreeBSD is a Unix-like free operating system descended from AT&T UNIX via the Berkeley Software Distribution (BSD) branch through the 386BSD and 4.4BSD operating systems. It runs on Intel x86 family (IA-32) IBM PC compatible computers, Sun UltraSPARC, IA-64, AMD64, PowerPC, ARM and NEC PC-9801 architectures along with Microsoft's Xbox. Support for other architectures is in varying stages of development. The DEC Alpha architecture was previously supported between releases 4.0 and 7.0, with the code being removed for FreeBSD 7.0. FreeBSD currently has more than 200 active developers and thousands of contributors.

FreeBSD has been characterized as "the unknown giant among free operating systems." It is not a clone of UNIX, but works like UNIX, with UNIX-compliant internals and system APIs. FreeBSD is generally regarded as reliable and robust.

FreeBSD is developed as a complete operating system. The kernel, device drivers and all of the userland utilities, such as the shell, are held in the same source code revision tracking tree, whereas with Linux distributions, the kernel, userland utilities and applications are developed separately, then packaged together in various ways by others.

History and development

FreeBSD's development began in 1993 with a quickly growing, unofficial patchkit maintained by users of the 386BSD operating system. This patchkit forked from 386BSD and grew into an operating system taken from U.C. Berkeley's 4.3BSD-Lite (Net/2) tape with many 386BSD components and code from the Free Software Foundation. The first official release was FreeBSD 1.0 in December 1993, coordinated by Jordan Hubbard, Nate Williams and Rod Grimes with a name thought up by David Greenman. Walnut Creek CDROM agreed to distribute FreeBSD on CD and gave the project a machine to work on along with a fast Internet connection, which Hubbard later said helped stir FreeBSD's rapid growth. A "highly successful" FreeBSD 1.1 release followed in May 1994.

However, there were legal concerns about the BSD Net/2 release source code used in 386BSD. After a lawsuit between UNIX copyright owner at the time Unix System Laboratories and the University of California, Berkeley, the FreeBSD project re-engineered most of the system using the 4.4BSD-Lite release from Berkeley, which, owing to this lawsuit, had none of the AT&T source code earlier BSD versions had depended upon, making it an unbootable operating system. Following much work, the outcome was released as FreeBSD 2.0 in January 1995.

FreeBSD 2.0 featured a revamp of the original Carnegie Mellon University Mach virtual memory system, which was optimized for performance under high loads. This release also introduced the FreeBSD Ports system, which made downloading, building and installing third party software very easy. By 1996 FreeBSD had become popular among commercial and ISP users, powering extremely successful sites like Walnut Creek CD-ROM (a huge repository of software that broke several throughput records on the Internet), Yahoo! and Hotmail. The last release along the 2-STABLE branch was 2.2.8 in November 1998. FreeBSD 3.0 brought many more changes, including the switch to the ELF binary format. Support for SMP systems and the 64 bit Alpha platform were also added. The 3-STABLE branch ended with 3.5.1 in June 2000.