Nothing ruins goods shot more than missing the focus. Modern cameras are very good at helping you get focus on your subject, and keeping the focus. But you have to use the right Auto Focus mode.
AF-S or AF-C, that is the question. The answer is, "it depends". AF-S, or Auto Focus Single is used when your subject is not moving in your frame. It is great for landscapes, taking portraits or travel photography. In this mode, when your shutter is pressed half way down, the focus is locked. Great for "focus and recompose" Use AF-C, or Auto Focus Continuous when your subject is moving, like in the sequence below. As long as you continue to hold down the shutter down, your camera will try to track he subject. Think action, like kids running around or a plane zooming past. Check out your camera manual to learn how to switch Focus Modes. Some cameras are better at AF-C than others. The newest models even have face and eye detection and tracking. Hmmm, sounds like a good idea for another Blog.
In this sequence, I used Auto Focus - Continuous to track this truck as it was moving toward me. Notice how the Ford emblem remains in focus between images? In AF-C, the camera tries to acquire focus as long as you keep the shutter down.
One more thing, If you are taking action photography, Back Button Focus is a MUST! Ahhh, another blog idea.