There is actually a pretty good, accurate, and comprehensive explanation in Wikipedia under "Pixel":


http://en.wikipedia.org/wiki/Pixel


The whole article is worth reading, and re-reading.


Here is a relevant sub-section copied from that Wikipedia "Pixel" entry:





<span>A pixel is generallythought of as the smallest single component of a digital image. The definition is highly context-sensitive. For example, there can be "printed pixels" in a page, or pixels carried by electronic signals, or represented by digital values, or pixels on a display device, or pixels in a digital camera (photosensor elements). This list is not exhaustive, and depending on context, there are several terms that are synonymous in particular contexts, such as pel, sample, byte, bit, dot, spot, etc. The term "pixels" can be used in the abstract, or as a unit of measure, in particular when using pixels as a measure of resolution, such as: 2400 pixels per inch, 640 pixels per line, or spaced 10 pixels apart.


The measures[url="http://en.wikipedia.org/wiki/Dots_per_inch][i]dots per inch[/i][/url][i](dpi) and[/i][url="http://en.wikipedia.org/wiki/Pixels_per_inch][i]pixels per inch[/i][/url][i](ppi) are sometimes used interchangeably, but have distinct meanings, especially for printer devices, where dpi is a measure of the printer's density of dot (e.g. ink droplet) placement.[/i]<sup class="reference" id="cite_ref-6"][url="http://en.wikipedia.org/wiki/Pixel#cite_note-6]<span>[i][[/i][i]7[/i]<span>[i]][/i][/url]</sup>[i]For example, a high-quality photographic image may be printed with 600 ppi on a 1200 dpi inkjet printer.[/i]<sup class="reference" id="cite_ref-7"][url="http://en.wikipedia.org/wiki/Pixel#cite_note-7]<span>[i][[/i][i]8[/i]<span>[i]][/i][/url]</sup>[i]Even higher dpi numbers, such as the 4800 dpi quoted by printer manufacturers since 2002, do not mean much in terms of achievable resolution.[/i]<sup class="reference" id="cite_ref-8"]<span>[9<span>]</sup>


The more pixels used to represent an image, the closer the result can resemble the original. The number of pixels in an image is sometimes called theresolution, though resolution has a more specific definition. Pixel counts can be expressed as a single number, as in a "three-megapixel"digital camera, which has a nominal three million pixels, or as a pair of numbers, as in a "640 by 480 display", which has 640 pixels from side to side and 480 from top to bottom (as in aVGAdisplay), and therefore has a total number of 640 &times; 480 = 307,200 pixels or 0.3 megapixels.


The pixels, or color samples, that form a digitized image (such as aJPEGfile used on a web page) may or may not be in one-to-onecorrespondencewith screen pixels, depending on how a computer displays an image.


I think that one place where a lot of people get confused is that they forget (or don't realize) that neither "Dots Per Inch" nor "Pixels Per Inch" says anything specifically about the physical size of that "Dot" or "Pixel". All it is saying is how many of these (whatever size) dots or pixels there are in one inch. Many people leap to the conclusion that if a printed image is at "600 DPI", that each Dot must therefore be 1/600 inch in diameter. Not necessarily true!


There's a lot more to this discussion, but start by reading the Wikipedia entry.


-Rockland Paul