What's the difference between FreeBSD and OpenBSD?

[Andy]

Apologies if I should have found the answer already, but it would appear
from both sites that xxxxBSD is a marvellous operating system, very secure,
efficient, etc, based on Berkeley Unix, etc. Both are free and maintained by
really skilled technical people, etc, but what is the difference between
them, why would one use one in preference to the other?