[gordon g]

He is right talking about printing output as Dpi. I think a lot of photographers these days confuse this with Ppi (pixels per inch).
Dots per inch is an output measure - a way of looking at print quality. Most modern injet photo printers can output 1440-2880 dpi - way above the quality of magazines and newsprint, which is where the 300dpi standard came from. For large prints on fine art paper, I find 1440 perfectly adequate. Using 2880 adds little to the print quality at normal viewing distances, but considerably slows down the printing process. Higher dpi gives finer graduations of tone and detail because the printer is using smaller dots, but that doesnt alter the number of pixels in a digital image.
Pixels per inch is an expression of the resolution of a digital image. 300 ppi is a better resolution than most people's eyes can see - 240ppi is adequate for most images. But it does depend on the output device - computer screens are often set at around 72-90ppi, so they just cant show high numbers of pixels per inch. There is little point therefore saving images at 300ppi to be viewed on a screen.
Ppi does have an influence on the output size of an image. If you have a 3000 by 3000 pixel image at 300ppi it will be 10 inches square. If it is output at 100ppi it will be 30 inches square. What it doesnt do is change the number of pixels an image actually consists of.
Clear as mud I expect, but I hope that helps!