The slightly over-simplified version goes like this:
OpenBSD is for firewalls, specialized routers, traffic management, and other internet-facing security applications.
FreeBSD is a reasonable general-purpose server BSD. It was among the first to have support for "really big" filesystems, but Linux has since caught up a bit. ZFS on FreeBSD is pretty rock-solid though.
NetBSD is intended to run on everything. Even toasters (http://www.embeddedarm.com/software/arm-netbsd-toaster.php).
There's a lot of overlap, and advocates for each will probably point out other things their OSes are good at.
In general, Linux has won the race towards user-friendliness. The recent systemd thing has caused (or will cause) the BSDs to fall even farther behind, because it is fundamentally incompatible with BSD (see point 15 on http://0pointer.de/blog/projects/the-biggest-myths.html), and more upstream software will begin to develop dependencies on systemd.
And, in general, Linux enjoys a greater amount of developer participation, which makes it faster-moving.
But, in general, the BSDs have produced more stable operating systems ideal for set-it-and-forget-it servers.
I have to regularly work with Debian, FreeBSD, and OpenBSD systems (and irregularly work with just about everything else, even the occasional Solaris). OpenBSD and FreeBSD kick the crap out of Linux when it comes to managing network traffic. But, for my personal machine, it's gotta be Debian.