Wikipedia
FreeBSD is a free Unix-like operating system descended from Research Unix via the Berkeley Software Distribution (BSD). Although for legal reasons FreeBSD cannot use the Unix trademark, it is a direct descendant of BSD, which was historically also called "BSD Unix" or "Berkeley Unix". The first version of FreeBSD was released in 1993, and today FreeBSD is the most widely used open-source BSD distribution, accounting for more than three-quarters of all installed systems running open-source BSD derivatives.
FreeBSD has similarities with Linux, with two major differences in scope and licensing: FreeBSD maintains a complete operating system, i.e. the project delivers kernel, device drivers, userland utilities and documentation, as opposed to Linux delivering a kernel and drivers only and relying on third-parties for system software; and FreeBSD source code is generally released under a permissive BSD license as opposed to the copyleft GPL used by Linux.
The FreeBSD project includes a security team overseeing all software shipped in the base distribution. A wide range of additional third-party applications may be installed using the pkgng package management system or the FreeBSD Ports, or by directly compiling source code. Due to its permissive licensing terms, much of FreeBSD's code base has become an integral part of other operating systems such as Juniper JUNOS and Apple's OS X as well as the operating systems running on Sony's PlayStation 3 and PlayStation 4.