Autofocus

Does exactly what is says on the box - you point the camera and it focuses on your subject.

Of course, it's a lot more complicated than that - there are several different ways a camera can do this - simpler "active" systems use an infra-red beam or sound waves to determine the distance to the subject, but these can be fooled by windows. Fancier cameras use "passive" autofocus, using sensors that actually look at the focus point and determine if it's in focus by looking at the contrast between different areas, but these can be fooled by areas of low contrast - you need to make sure you put a distinguishable object in the focus point.

High-end cameras like the D300 are able to track the focus point and keep it in focus even as it moves around the scene, and if the subject is moving towards or away from the camera, it can set the focus to where it will predict the subject will actually be when the shutter goes off - very clever.