Alvy Ray Smith, a former employee of Microsoft says that a pixel is not a little square, but a point sample, that exists only at a point. A pixel may contain all of the primary colours to make up a colour on the image.
"A pixel is a point sample. It exists only at a point. For a color picture, a pixel might actually contain three samples, one for each primary color contributing to the picture at the sampling point. We can still think of this as a point sample of a color. But we cannot think of a pixel as a square—or anything other than a point. There are cases where the contributions to a pixel can be modeled, in a low-order way, by a little square, but not ever the pixel itself."
http://alvyray.com/Memos/CG/Microsoft/6_pixel.pdf
Picture resolution
Picture resolution is essentially the amount of pixels going from top to bottom (height) and from left to right (width). An example of a well known resolution would be 1920x1080. 1920 is the width, and 1080 is the height. Picture resolution also depends on the camera you take the picture with. If you take a picture with a 5MP camera (mp stands for mega pixel) the image will be made up of 5 million pixels.
Image resolutionImage resolution is basically "image quality." If the resolution is high, the picture will be clearer and sharper because there is more information in the small space. If the resolution is lower, the picture will be less defined and more blurry looking.
The circle on the right has more pixels/information in a small space, which makes it look much sharper than the one on the left.
Intensity
A pixel that is part of an image and stored on a computer has a pixel value, which changes the brightness/intensity of the pixel. A common pixel format is know as a "byte image" where the pixel value can range between 0 and 255. 0 is black, and 255 is white, and the values in between are different shades of gray.
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