OK, so at least one person persuaded me to do this.
I'm going to make an attempt, given that some of you appear unprepared to read my friend Charles Maurer's article, to try to deliver a (relatively) short explanation of what constitutes a pixel (picture element). For this purpose I'm going to make some assumptions:
* I am here referring only to what are generally described as "Point & Shoot" (P&S) cameras, I am not referring to dSLRs even though the same principles also hold for them - except if they use the Foveon sensor, as in the Sigma SD14, when all bets are off but that's another subject entirely.
* All P&Ss use CCD (Charge Coupled Device) or CMOS (Complementary Metal-Oxide Semiconductor) sensors with an area of either 7.18mm x 5.32mm or 5.76mm x 4.29mm. The only exception to this is still vapourware so I'll ignore it. For purpose of this little exercise CCD and CMOS are interchangeable.
Do you know what RGB stands for? Red, Green and Blue, that is what your monitor employs to create the illusion of colour and so does your P&S. Of course RGB are very specific shades of those colours because between them they create all those other colours - including white and black (which are actually not colours at all). In a sensor (as described above) four cells (which the camera manufacturers, all of them, quite incorrectly and very misleadingly, count as a pixel) are employed to create a pixel and these cells are respectively sensitive to R, G, B and G. That collection of cells properly forms a pixel in a mosaic pattern so you can all divide by four to arrive at a proper pixel count for your own camera. This method is employed using the Bayer (after it's inventor) principle.
There is a problem here: because each pixel (as described above) creates a unique shade of colour the one immediately next to it may, often does, seem rather different to its neighbour so all these cameras use a blurring filter to overcome this problem. IOW, what you see is NOT what you get. The blurring algorithm is actually pretty good but it is nevertheless a blurring algorithm.
Now let's address the issue of printing and here there are also some difficulties because there is absolutely no point in trying to print something which the eye can't see - there is lots that the unaided eye can't see. For this purpose I am going to use A4 (297 x 210mm) as the benchmark. Note that A4 is more than double the size of a conventional, large, photograph.
Incidentally, and as an aside, when the Germans introduced this series of paper sizes to the world (in 1922) they were extremely clever about it because all the sizes are consistent with the proportion of 1 to the Square Root of 2. This is the only rectangle which allows you to reduce and enlarge an image (of whatever type) in direct proportion. No other rectangle does this and I am, quite simply, astounded that with a new era in photography the manufacturers of these sensors, all of them, chose to ignore this concept. They should, all of them, be hung, drawn and quartered for that.
The amount of processing power required to resolve an A4 image to print (trust me, I'm a retired printer) is 1,600 by 2,400 pixels (proper, real pixels - as described above) or 3.8 (real) megapixels, a far cry from what the camera manufacturers would have you believe and most of you wouldn't want to go beyond that size - if you did you'd be a contender for a decent dSLR which will almost inevitably have a bigger sensor than those described above. So even if your camera outputs more pixels than the above your eye will quite certainly not be able to discern any higher quality than that - except perhaps under a loupe, but who other than printers examines pictures under a loupe? Those extra pixels are only useful to you if you are editing in Photoshop or whatever which is why RAW is a desirable feature on cameras, especially for u/w use.
Hello? Just 3.8 megapixels will properly resolve into an A4 photograph? Just 1.9 megapixels will resolve into an A5 image (half A4 - 210 x 148.5 mm) which is rather closer to conventional photograph size but still a little larger? Do the camera manufacturers really want you to know that?
Remember what I said about sensor sizes? Here we come to noise, which is when a set of cells is so small as to be wellnigh useless:
Simply put it is just plain stupid to cram more cells into a finite space.
A (poor) analogy would be a double keyboard piano. Yup, lots more notes but will there ever be anyone that can properly control such a beast? Much more importantly:
the smaller the cell the more resistant it is to the acceptance of both wavelength (colour) and light, thus noise is created.
So, for example if you are looking at the purchase of a camera and the two you lust after have the same (theoretical) pixel count then, all other things being equal, you should choose the camera with the 7.18mm x 5.32mm rather than the one with the 5.76mm x 4.29mm sensor simply because the cells will be larger on the former camera. Now that's a really important consideration which should always be kept in mind.
More is not necessarily better than less. Less = larger cells = better sensitivity to light and wavelengths = larger pixels often of better colour saturation = well, less is actually more in many of the instances of P&S cameras - even sometimes dSLRs.
Caveat: This is a very simplified way to look at all this. To see the whole picture, read, and re-read my friend Charles Maurer's article:
TidBITS : Sense & Sensors in Digital Photography
which goes way beyond the scope of what I have written here.
If you go to that extent, you may also wish to read the other articles he has here.