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FreeBSD is a free and open-source software operating system. It is based on Unix, as opposed to merely being a clone in the vein of GNU/Linux. It has a relatively small (in comparison to GNU/Linux) but loyal userbase, many of whom are fiercely loyal to the traditional 'Unix Way'. Via its derivatives, OS X from Apple, and JUNOS from Juniper Networks, it has achieved a strong presence, if indirectly, on the desktop and on routers. It holds the largest userbase of all open-source operating systems that are directly derived from Unix.
FreeBSD also powers the popular NAS programs FreeNAS and NAS4Free.
Although not comparable directly to a GNU/Linux distribution - by virtue of FreeBSD not being a distribution (all of the base system is developed in a single tree rather than aggregated from elsewhere) - it shares a lot of similarities with Arch and Gentoo. Both Arch and Gentoo, for instance, have package management systems inspired by FreeBSD's Ports Tree - the (optional) Arch Build System, and Portage, respectively.