According to Wikipedia, JPEG stands for Joint Photographic Experts Group, the name of the committee that created the JPEG standard and also other still picture coding standards.
JPEG is a popular image file format. It is commonly used by digital cameras to store photos since it supports 224 or 16,777,216 colors. The format also supports varying levels of compression, which makes it ideal for web graphics.
How to Convert Images to JPEG format
Are iPhone photos JPEG?
With the “Most Compatible” setting enabled, all iPhone images will be captured as JPEG files, stored as JPEG files, and copied as JPEG image files too. This can help for sending and sharing pictures, and using JPEG as the image format for iPhone camera was the default since the first iPhone anyway.