DPI, PPI, Pixels, Megapixels and Resolution.

Pixel: The information stored for a single grid point in the image. The complete image is a rectangular array of pixels.

Resolution is a measure of graphics that is used to describe what a printer can print, a scanner can scan, and a monitor can display. It is generally describes as PPI or DPI. Both are used interchangeably, although DPI generally refers to an output devices capabilities. DPI (dots per inch) and PPI (pixels per inch) are both terms that describe how much digital information (pixels) your image has per square inch.

The standard resolutions are:
Screen/Monitor viewing 72 ppi
Photo Print 150 ppi
Press Quality Print 300 ppi

Another important term in describing resolution is resampling (The process of changing the pixel dimensions in an electronic photo to adjust image size). Resampling will decrease the quality of your image. It is very important not to scan your image at to high or low of a resolution, but it is better to scan the image too high than too low.

MegaPixel: One million pixels. When the length times width of a pixel array reaches one million, its resolution is then described in MegaPixels. 1,300,000 pixels equals 1.3 MegaPixels. It was derived to help digital camera buyers more readily understand the resolution of a camera. A more accurate measure of an image's resolution is its pixel count.