Autofocus can be your best friend in most shooting situations, but below are a few times when it’s wise to switch to Manual Focus.
Zeroing in on one subject:** When you want to focus on a particular object that the autofocus system doesn’t readily lock in on, focus manually. Most autofocus systems can be aimed, but it might simply be faster to do the focusing yourself.
Shooting action pictures:** In some action situations, autofocus systems can be fast, but they might focus at lighting speed on the wrong subject. Sports photographers can sometimes use manual mode to focus ahead of time where they know the action will be, dispensing with the uncertainty.

Avoiding total confusion: Autofocus systems are totally confused by some kinds of situations, such as subjects posed against plain backgrounds.
Photographing through glass:** Sometimes an autofocus system fixates on the glass itself rather than the subject behind it, so you might be better off focusing manually.

Adapted from Digital SLR Cameras & Photography for Dummies by David D. Busch (Wiley, 2007, $30)