Metadata is more data about data. In most common document types, embedded within a file is more information, typically hidden from casual viewing. This hidden data is used by the computer programs to provide accurate processing information, i.e. what version of software was used to create the document, how the file is encoded, and often who created it.
In the case with many popular image/picture formats, the list of possible metadata is quite extensive. With the expanded options for metadata in JPEG images, we have the ability to record the photographer, camera settings (ISO, Aperture, Flash, lens type), processing software and location.
The metadata in images is often retained by default by desktop image processing software and many online photo storage websites. This information is often valuable to the photographer, as well as the website provider for demographic information. Of course, location is also one of the options available for storage in a JPEG image...