Knowing and controlling exactly which part of a scene your camera is focusing on is very important, otherwise you're letting the camera decide what should be in sharp focus. And as we all know, cameras are pretty lousy at making creative decisions on their own.
All digital cameras have focus indicators in the viewfinder that show you where the camera is trying to focus. Simpler cameras often have just one central indicator that sits smack in the center of the viewfinder, while more advanced cameras typically have several separate indicators. The central indicator is fine if you tend to place important subjects in the center of the frame (which is rarely the most pleasing or creative place to put them--so avoid it if you can) or if the entire subject is equidistant--if you're shooting the side of a barn, for example.
You have far more creative focusing options if you have multiple indicators because they take in a broader area of the frame and, in many cameras, you can select which indicators you want to use. My Nikon D90, for example, has eleven different focus indicators and you can either let the camera decide which ones to use or you can use a custom setting to tell the camera what is the most important area of the frame.
The key thing with all types of focusing systems is to know where the camera is focusing--and because these AF systems are so reliable most of the time, it's easy to get fooled into thinking the camera always knows what you want to focus on. I was shooting this iron fence along a river walk, for example, and I had the indicators in the "auto" mode which meant the camera was choosing where it would focus. Unfortunately, as you can see in top photo, it focused on the river and not the fence! All I had to do in this case was to move the viewfinder over a hair until the indicators were over the edges of the fence. Instant sharp focus.
Don't take focus for granted--as you can see from the shot above, sometimes the camera makes some pretty wild (and wrong) decisions.
3 days ago