Resolution has at least two meanings.
- The correct meaning, used by physicists, biologists, and the scientific world in general, refers to the number of objects within a certain defined physical space. So in the publishing world it usually refers to Dots Per Inch (dpi), Pixels per Inch (ppi) , Lines Per Inch (lpi) or some other similar measurement.
- The incorrect but very heavily used meaning: which is 'the size of an image or display.'
To say a display's resolution is 640 pixels by 480 pixels is common, but not really correct. Now if you said it was showing those dimensions at 72dpi, then you'd be correct.
"Now what is the difference between dpi, ppi and lpi?"
- Dots refer to spots of ink, toner, or some other colorant on a piece of paper. They are the smallest dot that can be applied by that device and are usually monochromatic (they are either there or not, you don't have shades to choose from).
- Pixels (which is short for "picture element") refers to spots which appear on monitor screens. They can be one of a multitude of shades.
What's the difference?
Well, a pixel (of many shades) cannot be represented by a dot (of one shade) on a printer. So the printer gathers a whole bunch of dots together into a "cell" and, by varying the number of dots applied in that cell, can simulate a number of shades. If you want to simulate more shades, you need more dots in each cell. The number of cells per inch is called lines per inch (lpi) and is obviously not as many as there are dots per inch. What this means is that if you want your printer to output a continuous-tone image, you are going to have far fewer lines per inch than the say 600 or 1200 dpi you to which you are accustomed.