FreeBSD is a free Unix-like operating system descended from AT&T UNIX via the Berkeley Software Distribution (BSD). It 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 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.
Third-party application software may be installed using various software installation systems, the two most common being source installation and package installation, both of which use the FreeBSD Ports system.
Most software that runs on Linux can run on FreeBSD without the need for any compatibility layer. FreeBSD nonetheless still provides a compatibility layer for several other Unix-like operating systems, including Linux. Hence, most Linux binaries can be run on FreeBSD, including some proprietary applications distributed only in binary form. Examples of applications that can use the Linux compatibility layer are StarOffice, the Linux version of Firefox, Adobe Acrobat, RealPlayer, Oracle, Mathematica, MATLAB, WordPerfect, Skype, Wolfenstein: Enemy Territory, Doom 3 and Quake 4 (though some of these applications also have a native version). No noticeable performance penalty over native FreeBSD programs has been noted when running Linux binaries, and, in some cases, these may even perform more smoothly than on Linux.
However, the layer is not altogether seamless, and some Linux binaries are unusable or only partially usable on FreeBSD. This is often because the compatibility layer only supports system calls available in the historical Linux kernel 2.4.2. There is support of Linux 2.6.16 system calls, enabled by default since 8.0 and available since 7.0. However, there is currently no support for running 64-bit Linux binaries.
Previous post: SUSE Linux distributions.