I'm a new FreeBSD user but I still don't know the differences between 9.2, 9.3, 10.0, 10.1?
Most has already been answered but still... Another good way (in my opinion anyway) to look at this is "stable" vs. "development". Please note that it's hardly as extreme as it may sound, but it does hold truth nonetheless.
I personally consider the
9.x branch as "stable" (note: not insinuating that
10.x is unstable of course). Meaning so much as: there is hardly any development going on here, most of the updates focus at security and not so much new features.
Within that line of thinking
10.x would be denoted as "development" for me (most certainly not 'unstable' or such). As mentioned by the others too: here you'll find the latest drivers, support for the latest hardware and its an ongoing effort.
Although you didn't mention it I'd still like to include it because at the time of writing it's still being supported:
8.x. I'd describe this as "old stable". Its comparable to the
9.x branch where development is concerned, and it basically only gets security updates.
Note that this isn't the official description of this trio, but I still there's some logic to it and might make it easier to keep in mind.
So yeah; for servers my preferred version is definitely
9.3 right now, but workstations or experimental environments are better off using
10.x.