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The internet is flooded with articles about DPI. And I had hard time to understand it fully. I did a lot of research and consulted designers to get the heck out of it and here is a concise explanation of what it is, what it is to do with print, web and photo editors.I hope you have already gone through those stuff as I would be using less words to fill the missing LEGO pieces.

DPI or Dots per inch is for Print. It has nothing to do with web. You should consider about it only when you have print requirement.

Significance of value of DPI for Printer:
First thing first we are talking here about DPI of Printer and should not be associated with DPI of image right now. DPI is used to describe the resolution number of dots per inch in a digital print and the printing resolution of a hard copy print dot gain. A dot matrix printer, for example, applies ink via tiny rods striking an ink ribbon, and has a relatively low resolution, typically in the range of 60 to 90 DPI. An inkjet printer sprays ink through tiny nozzles, and is typically capable of 300-600 DPI. A laser printer applies toner through a controlled electrostatic charge, and may be in the range of 600 to 1,800 DPI.

Printer gets information not in pixel but in physical dimension say printer gets command to print 1 inch by 1 inch rectangle data. A 300 dpi printer would look for 300 dots data in digital data received. Now if the density of digital data (ppi -pixel per inch) is less than 300 (approx since ppi is not exactly equal to dpi) then image quality will be low (300ppi image data will have greater information filled in one inch than 72ppi. Right.) The more the dots printer can print and more the ppi of image the better the result.

Note: Printer doesn’t scale images based on its PPI. It prints what it gets.

Significance of value of DPI for Image:
Now back to digital word, DPI actually has no meaning. Its loosely equated to PPI and is significant when comes to printing data. Lets understand PPI. Say you have to take a picture which of a  square 1inch wide. Now if you want a good quality image you would prefer to have finest detail of that picture captured. You can capture the data using 300 pixels (300ppi) of inch or say using only 72 pixels (72ppi). Which is better? No point for guessing. 300 ppi of-course. What will be the image dimension of these two images in digital world (physical dimension would be 1 inch).   Obviously a you need 300 pixels to display 330 pixels and 72 pixels for 72 pixels So image dimension would be 330 X 300 and 72 X 72 respectively.

Significance of DPI (or PPI) for Printing Software:
Printing/Editing software such as Photoshop utilizes this PPI information embedded in the image to determine the actual print size of image in physical word by equating PPI to DPI.  If not present depending on software used it assumes PPI to 72 or 96ppi. When the data is send to printer this densified data is used by printer to get the print quality.

All above is applicable for Bitmap data and not Vector. Vector doesn’t work on DPI/PPI. For example flash, svg, pdf are vector format. these format takes care of print quality when sending data to printer of all the “vector objects” inside them.