FreeBSD is developed together as an entire operating system. The kernel, all of the expected userland utilities such as the shell and the device drivers are held in the same source code revision tracking tree (CVS). This is in contrast to Linux, a similar and more well known free Unix-clone, which is developed as a kernel by one group, userland utilities by others such as the GNU project, and put together with applications into distributions that package all the parts together by others. As an operating system, FreeBSD is generally regarded as quite reliable and robust, and of the operating systems that accurately report uptime remotely, FreeBSD is the most common free operating system listed in Netcraft's list of the 50 web servers with longest uptime. A long uptime also indicates that no kernel updates have been deemed necessary, as installing a new kernel requires a reboot and resets the uptime counter of the system.

Initial development of FreeBSD was started in 1993, and took its sources from 386BSD. However, due to concerns about the legality of all the sources used in 386BSD, FreeBSD re-engineered much of the system with the FreeBSD 2.0 release in January of 1995 using the 4.4BSD-Lite release from the University of California, Berkeley. The FreeBSD Handbook includes more historical information about the genesis of FreeBSD.

Initially FreeBSD employed the BSD Daemon as its logo, but in 2005 a competition for a new logo was arranged. On October 8, 2005, the competition finished leaving the design by Anton K. Gural as the new FreeBSD logo. As always, the BSD Daemon is the FreeBSD Project mascot.

The last FreeBSD release before 6.x series is 5.4 [3] (released on May 2005). FreeBSD developers maintain (at least) two branches of simultaneous development: a -STABLE branch of FreeBSD, from which releases are cut about once every 4-6 months. The latest 4-STABLE release of FreeBSD is 4.11, this is the last of 4-STABLE releases. The first 5-STABLE release was 5.3. The last 5-STABLE release will be 5.5. The first 6-STABLE release was 6.0. The development branch, -CURRENT, contains aggressive new kernel and userspace features. If a feature gets stable and mature it is eventually backported ("MFC" - Merge from CURRENT in the FreeBSD developer slang) to the STABLE branch. FreeBSD's development model is described in-depth in an article by Niklas Saers.

The big difference in FreeBSD 5 was a major change in the low-level kernel locking mechanisms to enable better symmetric multiprocessor (SMP) support, releasing much of the kernel from the GIANT lock, sometimes referred to as Big Kernel Lock. It is now possible for more than one process to execute in kernel mode at the same time.

Other major changes include an m:n threading solution called KSE which is now the default threading (pthreads) library, starting with 5.3 (the creation of the 5-STABLE branch). The terminology m:n, where m and n are small positive integers, implies that m userland threads correspond to n kernel threads. Many other new features are security related. The TrustedBSD project was formed by Robert Watson for the express purpose of adding trusted operating system functionality to the FreeBSD operating system. An extensible mandatory access control framework (the TrustedBSD MAC Framework), filesystem Access Control Lists (ACLs) and the new UFS2 filesystem all came from TrustedBSD. Some of the TrustedBSD functionality has been integrated into the NetBSD and OpenBSD operating systems as well.

FreeBSD 5 has also significantly changed the block I/O layer with the introduction of the GEOM modular disk I/O request transformation framework, contributed by Poul-Henning Kamp. GEOM enables the simple creation of many kinds of functionality, such as mirroring (gmirror) and encryption (gbde). The recent release of FreeBSD 5.4 has confirmed the FreeBSD 5.x branch as a highly stable and well-performing release, albeit one with a long gestation period due to the large feature set.

FreeBSD 6.0 was released on November 4, 2005, and 7.0-CURRENT is under development. These versions continue the work on SMP and threading optimization, as well as additional work in the area of advanced 802.11 functionality, TrustedBSD security event auditing, etc. The primary release accomplishments of this release include the removal of the Giant lock from VFS, replacement of the libthr library with a better performing implementation of 1:1 threading, and the addition of a BSM audit implementation, called OpenBSM, created by the TrustedBSD Project which is heavily based upon the BSM implementation found in Apple's Open Source Darwin which has been released under a BSD-style license.

The FreeBSD Ports Collection provides an easy and consistent way of installing software ported to FreeBSD. It uses Makefiles laid out in a directory hierarchy, so software can be installed and deinstalled with the make command.

When installing an application, very little (if any) user intervention is required after issuing the initial command. In most cases, the application is automatically downloaded from the Internet, patched and configured if necessary, compiled, installed, and registered in the package database. Any dependencies on other applications or libraries a port may have are also installed for the user.

Each port, or software package, is maintained by a "port maintainer", an individual who is responsible for staying current with the latest software developments. Anyone is welcome to become a port maintainer by contributing their favorite piece of software to the collection. One may also choose to adopt and maintain an existing port that has no maintainership.

Due to the size of the Ports Collection, with new software being contributed on a regular basis [4], many users will never have to look elsewhere for third party applications. As of January 12, 2006, there are over 14,000 pieces of software available in the collection.

Precompiled (binary) ports are called "packages", and are available for download.

The pre-compiled packages are generally separated into three sections, one intended for use with the 4.x code branch (4.9-RELEASE, 4.10-RELEASE, 4.11-RELEASE, 4.11-STABLE) another for use with the 5.x branch, and a third for use with the new 6.0-RELEASE branch. In almost all cases, a package created for the 4.x branch of FreeBSD can be installed in versions 5.2.1-RELEASE and beyond without difficulty.

If you know the name of the package you would like to install, the installation can be entirely automated by passing the package name to the pkg_add -r command. The appropriate package for your release will then be downloaded and installed along with any software dependencies it may have. By default, packages will be downloaded from the main FreeBSD distribution site.

FreeBSD provides binary compatibility with several other Unix-like operating systems, including Linux. The reasoning behind this is generally attributed to being able to run applications developed for Linux, often commercial, that are only distributed in binary form and thus cannot be ported to FreeBSD without the will of those who control the source code.

In a nutshell, it allows FreeBSD users to run a majority of the applications that are only distributed as Linux binaries. When compared to the vast number of native applications available for FreeBSD using the Ports Collection, these applications are in the minority. Applications used under the Linux compatibility layer include StarOffice, the Linux version of Netscape, Adobe Acrobat, RealPlayer, VMware, Oracle, WordPerfect, Skype, Doom 3, Quake 4, the Unreal Tournament series, Beonex and so on. Generally, there seems to be no noticeable performance penalty when running Linux binaries as opposed to native FreeBSD ones.

Though there are many applications that run flawlessly under the compatibility layer, it should be noted that the layer is not complete, thus rendering some Linux binaries unusable on FreeBSD or limiting their functionality, possibly because this compatibility layer only supports the system calls of Linux Kernel 2.4.2, a historic release. One example of this is Cedega, TransGaming's product to run Microsoft Windows games on Linux. Its usage is largely crippled at this time due to an incomplete compatibility layer. There has, however, been limited success in using it to run games on FreeBSD

As with its sister operating systems, OpenBSD, NetBSD and DragonFlyBSD, the code in FreeBSD is released under the terms of a variety of licenses. All of the kernel code and most newly created code is released under the terms of the two-clause BSD license, which allows everyone to use and redistribute FreeBSD as they wish, as long as they do not remove the copyright notice and the BSD license itself. This does not prohibit redistribution under another license. There are also parts under the GPL, LGPL, ISC, three-clause BSD license and four-clause BSD license.

A range of free and proprietary products are directly and/or indirectly based on FreeBSD, including Juniper routers, Apple's Mac OS X, Nokia's firewall operating system, and countless others. Other operating systems, including VXWorks (a Real-time computing operating system found running on anything from Linksys routers to the Common Core System of Boeing's 787 Dreamliner) and even Linux, derive critical technologies from FreeBSD, illustrating the broad reach of FreeBSD technology.

Other works derived or created in whole or part from FreeBSD technologies:

* Darwin, the core of Apple's Mac OS X, borrows heavily from FreeBSD, including its virtual file system and network stack kernel code, as well as large portions of its "BSD" userspace component. Apple regularly continues to integrate new code from, and contribute changes to, FreeBSD.
* OpenDarwin, a completely separate from Apple project that was originally based on Darwin, and hence bears considerable part of FreeBSD programming as heritage. Though the operating system is independently controlled, the developing community tries to maintain OpenDarwin's compatibility with Darwin/Mac OS X.
* PC-BSD is a FreeBSD distribution designed for desktop use with simplified package management and a graphical installer
* BSDeviant is a live FreeBSD distribution that can fit on one Mini CD-R.
* ClosedBSD, a distribution aimed at firewall and NAT services.
* GNU/kFreeBSD
* Debian GNU/kFreeBSD, a derivative version based on GNU userland is currently being developed by Debian.
* Ging is a live CD distribution based on Debian GNU/kFreeBSD, hence indirectly based on FreeBSD.
* DragonFly BSD is a fork from FreeBSD 4.8 that uses an alternative multi-processor synchronization strategy from that chosen for FreeBSD release 5. It features a threaded message passing system similar to that found in microkernels.
* Firefly BSD is a commercially-supported operating system based on the experimental DragonFly fork of FreeBSD.
* The FreeSBIE project is producing live CD distributions of FreeBSD, similar to the Knoppix distribution of Linux. The FreeSBIE project also includes a toolset to help you make your own FreeSBIE live file systems and CDROM's.
* Frenzy is another FreeBSD based live CD, mainly oriented towards Russian speaking users.
* PicoBSD is a one-floppy version of FreeBSD.
* m0n0wall is a FreeBSD-based embedded firewall package.
* pfSense is a firewall/router based on m0n0wall and FreeBSD.
* FreeNAS is NAS server based on FreeBSD/m0n0wall.
* TrustedBSD is a set of trusted operating system extensions for FreeBSD, including support for mandatory access control, event auditing via OpenBSM, access control lists, and a variety of other security features. Many of these features are now present in the main-line FreeBSD distribution.
* Gentoo/FreeBSD is a Gentoo/Linux port under FreeBSD's operating system.
* DesktopBSD is a UNIX-like, desktop-oriented operating system based on FreeBSD. Its goal is to combine the stability of FreeBSD with the ease-of-use of KDE, its default graphical user interface.