In contrast to the numerous Linux distributions, there are only four major open source BSDs. Each BSD project maintains its own source tree and its own kernel. In practice, though, there appear to be fewer divergences between the userland code of the projects than there is in Linux.
It is difficult to categorize the goals of each project: the differences are very subjective. Basically,
FreeBSD aims for high performance and ease of use by end users, and is a favourite of web content providers. It runs on a number of platforms, including i386 based systems (“PCs”), systems based on the AMD 64-bit processors, UltraSPARC® based systems, systems based on Compaq's Alpha processors and systems based around the NEC PC-98 specification. The FreeBSD project has significantly more users than the other projects.
NetBSD aims for maximum portability: “of course it runs NetBSD”. It runs on machines from palmtops to large servers, and has even been used on NASA space missions. It is a particularly good choice for running on old non-Intel hardware.
OpenBSD aims for security and code purity: it uses a combination of the open source concept and rigorous code reviews to create a system which is demonstrably correct, making it the choice of security-conscious organizations such as banks, stock exchanges and US Government departments. Like NetBSD, it runs on a number of platforms.
DragonFlyBSD aims for high performance and scalability under everything from a single-node UP system to a massively clustered system. DragonFlyBSD has several long-range technical goals, but focus lies on providing a SMP-capable infrastructure that is easy to understand, maintain and develop for.
There are also two additional BSD UNIX operating systems which are not open source, BSD/OS and Apple's Mac OS® X:
BSD/OS was the oldest of the 4.4BSD derivatives. It was not open source, though source code licenses were available at relatively low cost. It resembled FreeBSD in many ways. Two years after the acquisition of BSDi by Wind River Systems, BSD/OS failed to survive as an independent product. Support and source code may still be available from Wind River, but all new development is focused on the VxWorks embedded operating system.
Mac OS X is the latest version of the operating system for Apple Computer Inc.'s Macintosh® line. The BSD core of this operating system, Darwin, is available as a fully functional open source operating system for x86 and PPC computers. The Aqua/Quartz graphics system and many other proprietary aspects of Mac OS X remain closed-source, however. Several Darwin developers are also FreeBSD committers, and vice-versa.
http://www.freebsd.org/doc/en/articles/explaining-bsd/index.html
