http://www.scantips.com/basics01.html

This is a pretty good beginners intro to the concept of dpi, lpi, spi, and the difference between screen and print resolutions.

The navigation is a little difficult.

Reasonable explanation here:
http://graphicssoft.about.com/od/glo...resolution.htm

and more here:
http://graphicssoft.about.com/cs/dig...ixelsprint.htm and here: http://desktoppub.about.com/cs/inter...resolution.htm

I don't like that first one. WAY to general. By that definition I use "low-res" screen images for print all the time and my large format stuff would only need a 300dpi scan.
Not enough info.

The second doesn't relate printer dpi to image resolution lpi (or dpi in Photoshop) and generalizes 'what most people need'

The third was my second choice. Scantips has fewer ads.

From one of my seminars on DPI

I cannot explain resolution without talking about history. I certainly couldn't explain the process without having this be in book form.

Here is the short incomplete answer...

DPI or Dots Per Inch was traditionally used in Printing as the number of rows of dots per inch in the screening process. Newspapers used 85 DPI, Web offset used 133 DPI, Commercial Printers used 150, 175 or 200 DPI. When the Computer revolution hit our industry. LPI or Lines Per Inch determined the amount of lines it would take to create one single dot in one row of dots of the Dots Per Inch process. This is where the dot resolution comes from. This led to much confusion in our industry as people started referring to LPI as DPI. To add more confusion, Pixels Per Inch or PPI was originally used to describe the resolution of photos. In Desktop Publishing, however, PPI has been called DPI. Most software programs have an area that shows resolution as DPI regardless of what it is referring to.

Before computer hard drives, photo resolution from the scan to film process (in today’s terminology) was 3600 DPI (or greater). As printing evolved with computer technology, 3600 DPI photos were to large (or impossible) for the drives to hold cost effectively. The answer was to find the lowest possible resolution that could be printed before photos started to pixelize. 300 DPI was found to be that breaking point. It was also noted that changes in resolution effected the physical image size so the DPI could not be reduced for cost effective storage. (Like today’s Web pages) Once the pixel information was removed, it could not be restored.

Hope this helps

I'm just doing some research on this very subject. Your milage may vary, but I found some of the following useful.

http://www.just-stuart.com/photogs/DPI_Confusion

http://www.luminous-landscape.com/tu...solution.shtml