DPI stands for "dots per inch," and it refers to the density of the ink when you print. 300 dpi printed on quality paper will look pristine to the eye when held at typical reading distance. 600 dpi is about what you get from a professionally printed photograph, and fine art prints might go as high as 1200. A wall poster is frequently somewhere around 150 - 200, since it's meant to be viewed from a few feet away. If you set your word processor to print in draft mode, you'll probably get 100. High quality mode will likely be either 200 or 300. Note that the quality of the paper is also an important variable. If you try to print 300 to ordinary copy paper you'll saturate it, and the colors might bleed.

If you start talking about images that will only be viewed on the screen, then dpi is meaningless, as there is no way of knowing what the actual dimensions of the viewer's screen are, nor the density of pixels on that screen. The only thing that matters for online images is actual pixel dimensions. You will hear people insist that images designed for the screen are 72 dpi, but that's a holdover from the earliest Macintosh computers, which were designed in such a way as to make each pixel the same physical size as a point in print production (72 points = 1 inch). The truth is that if the image is only electronic, then dpi can be set to anything, and it won't change how the image looks.

The thread I mentioned is here: http://www.cartographersguild.com/tu...explained.html