What is the significance of these different width, height and resolution parameters?

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Published on 2013-07-28T20:02:04Z Indexed on 2013/10/17 22:01 UTC
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images

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file-format

An image with a pixel resolution of 640 x 480 has additional dimension and resolution parameters according to exiftool. I'm unsure what they mean.

Why are the X / Y Resolution parameters the same?
Should they not reflect the pixel dimensions of the image?
What does Exif Image Size mean and how is it different from the pixel dimensions?
What is the focal plane?
Does it have any relation to the device used to capture this image?

$ exiftool  evil1.jpg | egrep 'Width|Height|Resolution'
X Resolution                    : 180
Y Resolution                    : 180
Resolution Unit                 : inches
Exif Image Width                : 400
Exif Image Height               : 300
Focal Plane X Resolution        : 8114.285714
Focal Plane Y Resolution        : 8114.285714
Focal Plane Resolution Unit     : inches
Image Width                     : 640
Image Height                    : 480

If needed, the original image can be obtained from:

 here=http://www.pythonchallenge.com/pc/return/evil1.jpg
 wget --user=huge --password=file $here

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