PPI (Pixels Per Inch) measures the pixel density of a digital image or display. For digital images, PPI describes how many pixels are packed into one inch of the image's dimensions. For screens, PPI describes the pixel density of the display — higher PPI means sharper, more detailed display. Apple's "Retina" displays have 227-460 PPI, significantly higher than standard 96 PPI displays.
PPI is important for print output: an image with 300 PPI printed at its native size produces high-quality printed output. Scaling a 72 PPI image up for printing results in pixelation. However, for web display, PPI is irrelevant — browser rendering is determined by pixel dimensions, not PPI metadata. A 2000×1500 pixel image displays the same on screen regardless of whether it has 72 or 300 PPI metadata.
Common PPI values: Web images conventionally use 72 or 96 PPI (historical screen resolution). Print photos need 300 PPI. Large format printing (posters, banners) can use 150 PPI since viewing distance is greater. Passport photos are typically specified at 300 PPI — use FileCurve's Passport Photo tool for automatic correct sizing.