FreeBSD is a free and open-source 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”.
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 only delivering a kernel and drivers, 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.
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, Apple’s Darwin (which is the base for macOS, iOS, watchOS, and tvOS operating systems by Apple), pfSense, the Nintendo Switch system software, and the operating systems running on Sony’s PlayStation 3 and PlayStation 4.