Focus means how sharp the image is. This can be used to emphasise a certain point - for instance by focusing on an object close to the camera or further away. This can create meaning in an image.
This video can not be played
A guide to using autofocus and manual focus
Depth of field means how much of the image is in focus. A photograph might have:
Depth of field is controlled by the aperture. A larger aperture means a narrower depth of field.
A guide to depth of field