FreeBSD is targeted at x86 running on desktops and servers, and is generally considered the most "complete" distribution for those use cases, with the most pack

FreeBSD is targeted at x86 running on desktops and servers, and is generally considered the most "complete" distribution for those use cases, with the most packages available for that platform, widest range of features, etc. NetBSD focuses very heavily on portability. They target lots of architectures, focus on getting cross compilation working well, and spend a lot of time worrying about doing things "right" rather than expediently (for what it's worth; this can be good or bad depending on how you look at it). Their package manager/ports system is known as "pkgsrc", and also focuses on portability, such that it also runs on other platforms like Linux, Mac OS X, and so on. If you want to try porting a BSD to your odd little MIPS-based microcontroller, NetBSD is your best bet. OpenBSD is focused extremely heavily on security. If you are paranoid, you want to run OpenBSD. If you are not paranoid, you should be. They are out to get you. Don't let them. And the other major BSDs that you didn't ask about: Dragonfly BSD is a sort of next-gen fork of FreeBSD, a bit more experimental. The biggest new feature is its HAMMER filesystem, which is in the class of modern filesystems like Btrfs and ZFS, supporting things like snapshots, checksumming of data and metadata, mirroring to backups, data deduplication, and the like. It also has other features like user-space virtual kernels, better parallelism, etc. PC-BSD is designed to be a user-friendly, desktop-oriented BSD, which manages applications as separate bundles with installers, more similar to how they work on Windows on Mac OS X, than a traditional BSD ports system or Linux package manager which tries to manage shared dependencies (a PC-BSD package comes with all of its dependencies, which are installed separately from those of other packages). OS X is a hybrid operating system, based on combining the Mach microkernel, FreeBSD, and some NetBSD tools, along with a proprietary UI layer and applications. It runs on desktops, laptops, and ARM-based smartphones. It is well known for its user-friendliness, tight integration between software and hardware, and draconian application development policies for its package management system, the App Store. Luckily, there are alternative ports systems available, such as MacPorts, Homebrew, and Cydia.