> What is the relationship between OpenBSD, FreeBSD, and NetBSD?
>
> And why would one choose one of those over Linux?
The BSDs each are a complete system (as opposed to just a kernel) so if you rephrase your question like "why would one choose one of those over Debian, or Ubuntu, or RHEL, or Slackware, or Gentoo, or Arch...", then maybe you begin to see some parallels.
There are all kinds of reasons that make people prefer one Linux distro over another, and similar reasoning would apply to the BSDs. However, the differences between the BSDs tend to be larger than the differences between select two Linux distros.
Just to give you a glimpse of my personal reasons for using OpenBSD (both on notebook/desktop as well as on servers), it's probably got something to do with the 30 years of evolution and turd polishing that has resulted in a really really consistent and well integrated system. The OS is reasonably simple, requires little configuration & maintenance, it's well documented, it does what I want and it does what I expect it to do. It's predictable. It's easy to live with. It helps that everything is documented in a man page, and the same group of developers is in charge of integrating and documenting everything (even if not all parts of the system originate from the OpenBSD project).
It's hard to be more specific than this without pointing at the thousand little things it does right but which you don't notice until you're accustomed to them and then try any distribution of Linux where everything feels a little bit "off", everything is constantly changing (how many init systems and audio systems and daemons and configs and package managers have been invented or re-imagined beyond recognition in the past 10-15 years?), documentation is spotty, and all these groups of developers with different backgrounds and different goals make their own different things that aren't integrate in a way that masks all these differences..
OpenBSD is very consistent, and remains "Unixy". It hasn't been influenced by ex-Windows users trying to turn it into a Windowsy desktop OS, or by ex-Windows enterprise users trying to turn it into a windowsy enterprise server, or by corps turning it into an embedded router/PoS/whatever OS, or by hipsters trying to turn it into a next gen smartphone OS, or by hipsters trying to turn it into a cool must have Awesome OS X desktop, or by overzealous programmers trying to turn every little thing into a not-so-little programmable & scriptable monster that communicates with other programmable & scriptable monsters so (in theory) the thing can be made to do anything and everything (if you can program it and it's overcomplicated design & flaws allow for it...).
That's not to say it cannot be tasked with much of the tasks people use any other OS to complete; it just does things in a rather specific way. If you happen to like a traditional Unixy way, then OpenBSD is a good choice. I cannot speak of the other BSDs because I haven't used them, but I hear their strengths are quite different.
The BSDs each are a complete system (as opposed to just a kernel) so if you rephrase your question like "why would one choose one of those over Debian, or Ubuntu, or RHEL, or Slackware, or Gentoo, or Arch...", then maybe you begin to see some parallels.
There are all kinds of reasons that make people prefer one Linux distro over another, and similar reasoning would apply to the BSDs. However, the differences between the BSDs tend to be larger than the differences between select two Linux distros.
Just to give you a glimpse of my personal reasons for using OpenBSD (both on notebook/desktop as well as on servers), it's probably got something to do with the 30 years of evolution and turd polishing that has resulted in a really really consistent and well integrated system. The OS is reasonably simple, requires little configuration & maintenance, it's well documented, it does what I want and it does what I expect it to do. It's predictable. It's easy to live with. It helps that everything is documented in a man page, and the same group of developers is in charge of integrating and documenting everything (even if not all parts of the system originate from the OpenBSD project).
It's hard to be more specific than this without pointing at the thousand little things it does right but which you don't notice until you're accustomed to them and then try any distribution of Linux where everything feels a little bit "off", everything is constantly changing (how many init systems and audio systems and daemons and configs and package managers have been invented or re-imagined beyond recognition in the past 10-15 years?), documentation is spotty, and all these groups of developers with different backgrounds and different goals make their own different things that aren't integrate in a way that masks all these differences..
OpenBSD is very consistent, and remains "Unixy". It hasn't been influenced by ex-Windows users trying to turn it into a Windowsy desktop OS, or by ex-Windows enterprise users trying to turn it into a windowsy enterprise server, or by corps turning it into an embedded router/PoS/whatever OS, or by hipsters trying to turn it into a next gen smartphone OS, or by hipsters trying to turn it into a cool must have Awesome OS X desktop, or by overzealous programmers trying to turn every little thing into a not-so-little programmable & scriptable monster that communicates with other programmable & scriptable monsters so (in theory) the thing can be made to do anything and everything (if you can program it and it's overcomplicated design & flaws allow for it...).
That's not to say it cannot be tasked with much of the tasks people use any other OS to complete; it just does things in a rather specific way. If you happen to like a traditional Unixy way, then OpenBSD is a good choice. I cannot speak of the other BSDs because I haven't used them, but I hear their strengths are quite different.
Oh, and it's Free.