FreeBSD is a free, open source, Unix-like operating system descended from AT&T UNIX via the Berkeley Software Distribution (BSD) branch through 386BSD and 4.4BSD. It runs on processors compatible with the Intel x86 family, as well as on the DEC Alpha, the UltraSPARC processors by Sun Microsystems, the Itanium (IA-64) and AMD64 processors. Support for the ARM architecture, MIPS and PowerPC architectures are currently in development.
History and development
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.
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, the Unreal Tournament series, and so on. It is also reported that in some situations, Linux binaries perform better on FreeBSD than they do under Linux.
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. 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. Most newly created code is released under the terms of the three-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 re-distribution under another license. There are also parts under the GPL, LGPL, ISC and the four-clause BSD license.