Nikon D300 Complete Manual page 386

It's very important to note the primary difference between
Single Servo and Continuous Servo AF. Nikon calls the Single
Servo AF mode "focus priority" for a reason—an image is not
taken until the camera achieves focus. If the conditions are
such that the camera can't manage to find focus—as
sometimes happens with fast moving off-center subjects in
low light—you won't be able to take a picture.
Continuous Servo AF is called "release priority" by Nikon,
meaning that the picture is taken immediately after fully
pressing the shutter release, regardless of whether or not focus
has yet been achieved. That doesn't mean the resulting
picture is out of focus, however. As I've already noted, usually
the camera has enough time to move the focus point on the
lens. Sometimes depth of field is enough to cover any focus
error. And sometimes you just get lucky. But under release
priority, there's no guarantee that you'll get an in-focus image
if you didn't first press the shutter release partway and hold it
there long enough for the camera to establish a focus point.
Note: Now that you've learned all that, know that you can reverse
the release priority and focus priority capabilities using
Custom Settings #A1 and #A2. My suggestion is to avoid
making those changes until you've mastered other aspects
of the autofocus system, and then only if you have a specific
situation that warrants it.
I sometimes mockingly refer to Single Servo AF as the Shutter
Frustration mode and Continuous Servo AF as the Focus
Frustration mode. That's because until you learn all the
nuances of how the Nikon AF system works and can
anticipate and avoid the things that keep it from achieving
focus, you'll be frustrated that the shutter release never quite
works in a timely fashion if you're in one autofocus mode and
you never quite get in-focus pictures when you're in the
other. If you always shoot in bright light, you might not
encounter those problems, but I strongly suggest to every
Nikon newcomer that they actually practice focusing in a
variety of conditions with the various camera settings.
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 386