FreeBSD is a Unix-like free operating system descended from AT&T UNIX via the Berkeley Software Distribution (BSD) branch through the 386BSD and 4.4BSD operating systems. It runs on Intel x86 family (IA-32) PC compatible computers, DEC Alpha, Sun UltraSPARC, IA-64, AMD64, PowerPC, ARM and NEC PC-98 architectures along with the Microsoft Xbox. Support for the MIPS architectures is under development.

FreeBSD 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. Among all operating systems which can accurately report uptime remotely, FreeBSD is the free operating system listed most often in Netcraft's list of the 50 web servers with the longest uptime. A long uptime also indicates no crashes have occurred and no kernel updates have been deemed needed, since installing a new kernel requires a reboot and resets the uptime counter of the system.

FreeBSD is developed as 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 (CVS), whereas with Linux the kernel, userland utilities and applications are developed separately, then packaged together in various ways by other groups as Linux distributions.