Focus and depth of field are related, and the advancing photographer should understand the relationship between them.
If you are used to using a digital compact, then focus may be something you take for granted. Things just are in focus. But if you are thinking of buying a DSLR or a film camera, then focus becomes much more important. In fact it is one of the primary creative tools the photographer has.
Lenses resolve the light emitted from objects in the world into an image that we can use. Sometimes, as in a telescope, this is for direct observation, and the focus is inside the eye; in photography, resolution is always on a plane behind the lens.