While some of the least expensive digital cameras can only focus automatically, most SLR digital camera features three different focus modes: manual, single auto focus and continuous auto focus. Let's cover them one at a time.
When focusing manually, the camera does not focus at all while you, the photographer, decide on the best focal point. You set the different buttons or rotate a focusing ring on the camera lens to focus. This feature is for those who like to have complete creative control of the finished product and this is the best focus mode for the experienced.
In the single auto focus mode, the camera automatically itself either when you press the shutter button fully to take the picture or half way to lock the focus. This mode is useful when you are taking photographs of static objects. More... / Hide...
In continuous auto focus the camera incessantly focuses on the objects in your viewfinder. While the camera is in this mode, it continuously corrects the focal point as the distance between the subject and the camera varies. This mode is useful when you take photos of moving objects such as a race car during a race or airplanes in an air show. You can press the shutter release half way down and point the camera at the object and follow it without pause. The camera will unceasingly keep the subject in focus.
There are both advantages and disadvantages to automatic and manual focus modes. To use them to your advantage, it is important to understand how they work and what they were designed for. After that, you should experiment taking photos with different focus modes in various situations and find out how the camera performs. Only then will using the best focus mode for each shooting situation become second nature for you.