Focus - the basics
One of the things most prominent in an image is whether the subject is or is not sharp. If parts of an image are blurred your eye is immediately drawn to the part that is in focus. Although most digicams use autofocus there are a few things to consider to achieve good results.
There are two systems of autofocus.
Active autofocus systems which send out an infrared signal that is reflected by the subject, and passive autofocus systems that are able to calculate the subject's distance on the basis of light and contrast in the subject.
For both systems keep the subject inside the focus frame of your viewfinder before pressing the shutter halfway to lock focus. Then recompose the image and press the shutter completely.
Focus is also influenced by camera or subject movement.The former - probably the major cause of unsharp images - can be eliminated by holding the camera steady before pressing the shutter smoothly or by using a tripod. The latter can be rectified by choosing a faster shutter speed or panning the camera with the subject. Telelenses will require a fast shutter speed to eliminate movement. The rule of thumb here is not to use shutter speeds slower than the lens' focal length.