These should illustrate a few of the points.
Here is the skyline of Boston shot from a balcony along the Charles River:
Lots of foreground, and not much of the skyline. So, let's crop the photo and blow it up, so we get a better look at the Citgo sign:
But as I said, eventually if you blow a digital image up too much, the picture breaks into individual pixels, which looks like this:
A camera with more megapixels helps you get higher quality blow-ups. This is the Canon SD-500 reviewed for the article:
Note the slight wedge shape and rounded edge on the left. That, says a Canon spokesperson, is to make room for the 7.1 megapixel sensor.
A smaller and less expensive alternative is the 5 megapixel SD-400. Note that it is perfectly rectangular:
