Image files created in a camera are really data files. Most of the data tells the computer how to display the image in an application.
Non-image data is also stored in an image file. This non-image data is information about the file itself and its creation. For example the width and height in pixels is data stored to describe the image screen size. Other data stored about the file includes the ISO, Aperture and Shutter speed of the camera when the photograph was taken. This is not the data of the image, it is data that describes the image and how the image was created.
Non-image data that describes the file itself is called metadata. In general terms, metadata is data describing other data. Metadata fields are closely specified in a ‘schema’ which sets out what goes in each field.
In photography metadata stored in image files tells us about the image, the camera, what settings were used when the shot was taken and lots of other data. It is all stored in a standard format called EXIF (Exchangeable Image File Format). Other formats/schema of metadata may be included in an image file. IPTC data is an example.