DPI (Dots Per Inch) is a measure of print resolution — the number of ink dots a printer places per inch of paper. Higher DPI means more dots per inch, resulting in finer detail and smoother gradients in printed output. Standard printing uses 300 DPI for high quality; newspapers use 85-100 DPI; laser printers typically use 600-1200 DPI for text.
DPI is often confused with PPI (Pixels Per Inch), which measures the pixel density of a digital image. When someone says "this image is 300 DPI," they usually mean 300 PPI — the image has 300 pixels per inch of its dimensions. To calculate pixel dimensions from physical size: multiply dimensions in inches by DPI. A 4×6 inch photo at 300 DPI = 1200×1800 pixels.
For web use, DPI is irrelevant — screen display is determined by pixel dimensions and screen resolution. A 72 DPI vs 300 DPI image with the same pixel dimensions looks identical on screen. DPI metadata in JPEG files only matters when sending to a printer. When compressing images for web, ignore DPI — focus on pixel dimensions instead.