What is a Megapixel?
This camera has 10, that one has 15, but really, what IS a megapixel?
The Pixel
"Pixel" is short for "picture element" It is the smallest, most fundamental part of an image. The pixel isn't made of anything smaller - it is just a square of a single colour.
The pixel is the building block of photos. In the same way as you are made of atoms which you can't see, when many tiny pixels are put together, you don't see them but a complete and seamless image.
Here's the waterfall from the centre photo at the top of the site zoomed right in... see the pixels?
Ok, but what is a megapixel? 1 megapixel or "1MP" is one million pixels. Let me explain why...
"Mega" is a scientific term used to denote an order of magnitude. For example, let's take the metre. You are familiar with the kilometre as well - a kilometre is one thousand metres. (1,000 is three orders of magnitude, i.e. 3 factors of 10.)
Well, these go in factors of 1,000, so a "megameter" would be 1,000 km or 1,000,000, if we used it. Similarly with the gram. 1,000g = 1kilogram. 1,000 kg would be a megagram except we instead call it a "metric ton", but we get explosives measured in kilotons and megatons of TNT.
So just remember that 1 megapixel is one million pixels.
Image Size
Say you have an image on your screen. You can measure its size in inches and centimetres, or you can do it in pixels. For example, an image can be 200px ("px" is short for pixel) wide and 100px tall.
How many pixels are there in total in this image? Just multiply the two sides together: 100px x 200px = 20,000 px in the image. The same system is true of the digital sensor in digital cameras.
My Canon 400D has a 10.1MP sensor, so it has 10,100,000 light-sensitive pixels on it. Picking one of my high resolution photos at random, its size is 3,888px wide by 2,592 px high. That gives a total number of 10,077,696px or 10.1 MP.
Now you understand what it means when a camera "is a 10MP camera or a 15MP camera". It simply refers to the size of the image. Bear in mind that your eye can't tell the difference above a certain resolution, they all look crystal clear. The main advantage to a huge MP camera is how large you can print a photo without it "pixelating" like the image above.
Pixelating is what happens when you enlarge an image so much that you can see the individual pixels with the naked eye, like the zoomed in version of my image above. It's generally regarded as a bad thing in photography!
Anyway, I hope that provides a decent answer to the question: what is a megapixel?