Depth of field is the near to far sharpness in your pictures. A picture with shallow depth of field has only the main subject in focus, and everything in front of it and behind it is blurred. The opposite is a picture with deep depth of field, where everything is sharp from right up close […]
Circle of confusion
This is a concept used in calculating depth of field. When a point is slightly out of focus it's rendered as a circle instead, but as long as this 'circle of confusion' is small enough, it still looks like a point.