The biggest difference between JPEGs and PJPEGs is the way images are loaded. Images saved as baseline JPEGs (that is, “normal” JPEGs) are loaded line by line from top to bottom. In the case of progressive JPEGs, however, the entire image is shown immediately and loaded pixel by pixel. So a pixelated, blurry image gradually sharpens. While baseline JPEGs are loaded from top to bottom, PJPEGs can be said to load from back to front.
This has a positive effect on user experience since PJPEGs appear to load faster due to the lack of extra whitespace on the page. The following images illustrate how JPEGs and PJPEGs are loaded. In the case of baseline JPEGs, the pixels appear line by line, from top to bottom: