I have been considering entering one of my photos that actually turned out rather well (totally by accident, I assure you) in a local contest. Upon reviewing the prospectus for this contest (a "prospectus," I have learned, is a fancy term for a list of the contest rules), I read that I needed to electronically submit a 5x7 copy of my photo, and that the resolution should be 1200 dpi on the longest edge.
Hmm. I remembered from my printer-shopping days that "dpi" stands for "dots-per-inch." So, I checked my photo, and the resolution is about 580 ppi (pixels per inch). I was momentarily confused, and I did what I always do when I'm confused - I Googled it.
As it turns out, there are about as many explanations of "dpi" and "ppi" as there are photographers, and the words are often used interchangeably, but actually mean different things. After about two hours of reading, I finally came up with a way to explain this so that even I can understand it:
1) Dots per inch (DPI) when used literally applies to printers and scanners. 1200 DPI is a good minimum standard for most print jobs.
2) Pixels per inch (PPI) when used literally applies to digital images. 300 PPI is a good minimum standard for most online publishing.
3) Those who are more concerned with the image in PRINT will tend to pay more attention to the "1200" figure, where as those more concerned with the DIGITAL version of the image will tend to pay more attention to the "300I" figure. In either case, they are likely to use the term "DPI" and "PPI" interchangeably.
4) Photos that meet one of the minimum standards will meet the minimum standard of the other. HERE's a great DPI to PPI converter for those who need higher resolutions or want to be more exact.