JPG (or JPEG) is a raster image that is often used for photographs 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 image files to be as small as they can be so your site loads quickly, but large enough to still appear crisp and not pixelated. A JPG can’t 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 photographs on your website.
To edit this kind of resources, you will need an edition software like Adobe Photoshop, but there are plenty of online edition software that you can use to manipulate a JPG file.