Pixels!

The word itself is a play on "Picture Elements". It is directly related to the number of elements in the CCD sensor. Each pixel is represented by three values, one each for red, blue and green. This value can vary from 0-255. A combination of these three values gives a total of around 16 million ( actual value is 256*256*256 ) colors. Usually each camera manufacturer touts its Mega-pixel count the loudest. Here is a rough estimate of the biggest size picture you can usually take for each megapixel count.

3 Mega Pixel Cameras - Usually 2048 pixels (length) * 1536 pixels (height)

4 Mega Pixel Cameras - Usually 2272 pixels (length) * 1712 pixels (height)

5 Mega Pixel Cameras - Usually 2592 pixels (length) * 1944 pixels (height)