The JPG is a raster image that is designed for use on the web. JPGs can be optimized, when saving them out of photoshop, to find the perfect balance of small file size and high quality. On the web, you want your images files to be as small as they can be so your site loads quickly, but large enough to still appear crisp and not pixilated. A JPG can not have a transparent background so they are always in the shape of a rectangle or square with a solid background.

Best use = rectangle or square photos and images on your website.