Darwin is an open source POSIX -compliant computer operating system released by Apple Inc. in 2000. It is composed of code developed by Apple, as well as code derived from NeXTSTEP , BSD , and other free software projects.
Darwin forms the core set of components upon which Mac OS X , Apple TV , and iPhone OS are based. It is compatible with the Single UNIX Specification version 3 (SUSv3) and POSIX UNIX applications and utilities.
Darwin's heritage began with NeXT 's NeXTSTEP operating system (later known as OPENSTEP ), first released in 1989. After Apple bought NeXT in 1997, it announced it would base its next operating system on OPENSTEP. This was developed into Rhapsody in 1997 and the Rhapsody-based Mac OS X Server 1.0 in 1999. In 2000, Rhapsody was forked into Darwin and released as open-source software under the Apple Public Source License (APSL), and components from Darwin are present in Mac OS X today.
Up to Darwin 8.0, Apple released a binary installer (as an ISO image ) after each major Mac OS X release that allowed one to install Darwin on PowerPC and Intel x86 computers as a standalone operating system. Minor updates were released as packages that were installed separately. Darwin is now only available as source code,  except for the ARM variant, which has not been released in any form separately from iPhone OS. However, the older versions of Darwin are still available in Binary form.
Darwin is built around XNU , a hybrid kernel that combines the Mach 3 microkernel , various elements of BSD (including the process model, network stack , and virtual file system ),  and an object-oriented device driver API called I/O Kit . 
Some of the benefits of this choice of kernel are the Mach-O binary format, which allows a single executable file (including the kernel itself) to support multiple CPU architectures, and the mature support for symmetric multiprocessing in Mach. The hybrid kernel design compromises between the flexibility of a microkernel and the performance of a monolithic kernel .
Darwin currently includes support for both 32-bit and 64-bit variants of the Intel x86 processor used in the Mac and Apple TV as well as the 32-bit ARM processor used in the iPhone and iPod Touch . An open-source port of the XNU kernel exists which supports Darwin on Intel and AMD x86 platforms not officially supported by Apple. 
It supports the POSIX API by way of its BSD lineage and a large number of programs written for various other UNIX-like systems can be compiled on Darwin with no changes to the source code .
Darwin and Mac OS X both use I/O Kit for their drivers and therefore support the same hardware, file systems, and so forth. Apple's distribution of Darwin included proprietary (binary-only) drivers for their AirPort wireless cards.
Darwin does not include many of the defining elements of Mac OS X, such as the Carbon and Cocoa APIs or the Quartz Compositor and Aqua user interface , and thus cannot run Mac applications. It does, however, support a number of lesser known features of Mac OS X, such as mDNS Responder, which is the multicast DNS responder and a core component of the Bonjour networking technology, and launched , an advanced service management framework .
In July 2003, Apple released Darwin under version 2.0 of the Apple Public Source License (APSL), which the Free Software Foundation (FSF) approved as a free software license . Previous releases had taken place under an earlier version of the APSL that did not meet the FSF's definition of free software, although it met the requirements of the Open Source Definition .
The Darwin developers decided to adopt a mascot in 2000, and chose Hexley the Platypus , over other contenders, such as an Aqua Darwin fish , Clarus the Dogcow , and an orca . Hexley is a cartoon platypus who usually wears a cap resembling a demon 's horns. He carries a trident, similar to the BSD Daemon , to symbolize the daemon's forking of processes . Hexley was designed and copyrighted by Jon Hooper. Apple does not sanction Hexley as a logo for Darwin. 
Hexley's name was a mistake: it was originally supposed to be named after Thomas Henry Huxley , a 19th century English biologist who was a well-known champion of Charles Darwin 's theory of evolution (nick-named "Darwin's bulldog"). However, ignorance led not only to a mistake in Huxley's name, but who he was thought to be. The developers apparently thought he was simply Darwin's
assistant, when in fact he was a prominent biologist in his own right. By the time the mistake had been discovered, it was deemed too late to change, and the incorrect name "Hexley" was kept. 
This is a table of major Darwin releases with their dates of release and their corresponding Mac OS X releases.  Note that the corresponding Mac OS X release may have been released on a different date; refer to the Mac OS X pages for those dates.
The jump in version numbers from Darwin 1.4.1 to 5.1 with the release of Mac OS X v10.1.1 was designed to tie Darwin to the Mac OS X version and build numbering system. In the build numbering system of Mac OS X, every version has a unique beginning build number, which identifies what whole version of Mac OS X it is part of. Mac OS X v10.0 had build numbers starting with 4, 10.1 had build numbers starting with 5, and so forth (earlier build numbers represented developer releases). The point release number in the Darwin version is always the same as the second point number in the Mac OS X version. In the case of Mac OS X v10.1.1 (the version where the jump in version numbers was made), this was build 5M28 and the 10.1.1 release, from which a version number of 5.1 was derived. 
The command uname -r in Terminal will show the Darwin version number, and the command uname -v will show the XNU build version string, which includes the Darwin version number.
OpenDarwin was a community-led operating system based on the Darwin system. It was founded in April 2002 by Apple Inc. and Internet Systems Consortium . Its goal was to increase collaboration between Apple developers and the free software community . Apple theoretically benefited from the project because improvements to OpenDarwin would be incorporated into Darwin releases; and the free/open source community supposedly benefited from being given complete control over its own operating system, which could then be used in free software distributions such as GNU-Darwin .
On July 25, 2006, the OpenDarwin team announced that the project was shutting down, as they felt OpenDarwin had "become a mere hosting facility for Mac OS X related projects," and that the efforts to create a standalone Darwin operating system had failed. They also state: "Availability of sources, interaction with Apple representatives, difficulty building and tracking sources, and a lack of interest from the community have all contributed to this. The last stable release was version 7.2.1, released on July 16, 2004.
In 2007, the PureDarwin project was launched to continue where OpenDarwin left off, and is currently working to produce a release based on Darwin 9. There is a developer preview available, called "PureDarwin XMas", based on Darwin 9. This release has X11 , DTrace , and ZFS .  PureDarwin nano is another release of PureDarwin that is supposed to be minimalistic.
MacPorts (formerly DarwinPorts) and Fink are both well known projects to port UNIX programs to the Darwin operating system and provide package management . In addition, several standard UNIX package managers—such as RPM , pkgsrc , and Portage —have Darwin ports. Some of these operate in their own namespace so as not to interfere with the base system.
GNU-Darwin is a project that ports packages of free software to Darwin.
The Darwine project is a port of Wine that allows one to run Microsoft Windows software on Darwin. 
SEDarwin is a port of TrustedBSD mandatory access control framework and portions of the SELinux framework to Darwin.  It was incorporated into Mac OS X 10.5. 
The Darbat project is an experimental port of Darwin to the L4 microkernel family . It aims to be binary compatible with existing Darwin binaries. 
There are various projects that focus on driver support: e.g., wireless drivers,   wired NIC drivers    modem drivers,  card readers,  and the ext2 and ext3 file systems.