check out the faq here:
http://www.cs.duke.edu/~parr/photography/faq.html#dpi
Think it explains pretty well. Here's an extract:
The first thing that you need to understand about digital pictures is that they don't have an inherent DPI rating. That's right, the resolution of digital images is not measured in DPI.. Digital images are collections of pixels. The DPI of an image depends entirely on how you decide to display it or print it.
http://www.cs.duke.edu/~parr/photography/faq.html#dpi
Think it explains pretty well. Here's an extract:
The first thing that you need to understand about digital pictures is that they don't have an inherent DPI rating. That's right, the resolution of digital images is not measured in DPI.. Digital images are collections of pixels. The DPI of an image depends entirely on how you decide to display it or print it.