Resolution - Pixels
Before discussing what resolution is, you should first understand what makes up an image. The most basic thing that can be said about a digital image is that it's a bunch of dots. No matter how the picture is actually presented, all digital images start as being a series of dots. When you talk about how clear or how detailed a digital image is, you are referring to the resolution of the image. The resolution is either expressed in pixels (or megapixels or as dots-per-inch (dpi).
The resolution for cameras is measured in pixels. When you are setting up your camera to take pictures the setting are typically called "Fine/High", "Basic", "Normal". The actual pixel measurement is buried somewhere in your manual and is stated in pixel dimensions. Camera pixel measurements are measured in MegaPixels, this refers to a million dots. Scanner resolution is typically measured in dpi (dots per inch).
Once you have your image, there are 2 destinations for the image -- a computer monitor or a printer.
There are printers that allow you to plug your camera directly into the printer or plug your memory card directly into the printer bypassing your computer, but typically the initial destination of your digital image is your computer. If you refer to this Print Equivalent page, you'll note that a 3.3 Megapixel image taken on your camera is 2048 pixels wide and 1536 pixels high. This can be considered to be a typical image size since 3 megapixels is what most cameras sold have. This 2048x1536 image can become a very crisp 7x5 inch picture when printed when printed at at 300 dpi (dots per inch). The dpi rating represents the resolution on your printed images. Your printer is able to take the dots from the original image and move them closer together which increases the quality of your printed image. A good quality photo is around 300 dots per inch (200 looks pretty good too depending on the paper that you use).
The image resolution that your monitor can display is quoted in the same way that the height and width of the photo is quoted. Your computer monitor does not have the same capability that your printer does. Your computer monitor is relatively fixed in the number of dots that it is able to display (it cannot change the # of pixels it displays at will). Many users are most comfortable with a display resolution of 1024x768 or 800x600 -- this is the number of dots wide by the number of dots high that your monitor will be displaying. The typical PC actually displays your images at around 96 dpi, the typical Mac displays at 72 dpi.
When measuring DESKTOP PRINTER OUTPUT RESOLUTION, use dots per inch. DPI refers to the resolution of an output device like a laser printer or inkjet printer. A standard office laser printer is 300 dpi. A standard image setter is 2,540 dpi. When measuring MONITOR/SCANNER OUTPUT RESOLUTION, use pixels per inch. PPI is the resolution (or detail) of an image in a scanning or graphics program, and the resolution of computer monitors: The standards are, 72 ppi for Macintosh and 96 ppi for PCs.
To provide you with an illustration, the 3.3 megapixel image that you took on your camera (2048x1536) is going to be displayed on your 1024x768 monitor. The image is twice the size of your monitor resolution. 1 of 2 things will happen when you look at that picture; 1- the software will display the image as-is and you will see the top-left quarter of your image, or 2- your software will zoom in and display your image at "25%", which means that the software will display every 4th pixel. Keep in mind that the entire image is stored in the memory of your computer, it is just for display purposes that your software will do this. Please note, your image software DOES keep track of all of the dots in your image, but when the image is zoomed, the pixels are kept in memory and may not be displayed.