When a digital camera captures an image it records two things in the file that is saved – image data itself, and data about that data (confusing, isn’t it?). The “about the image” data is called “metadata”. The information it contains varies from camera to camera, but typically includes, amongst other things, information such as date, time, shutter speed, aperture, ISO and file type.
This information can be useful when trying to find photos later (because you can search for your images by date), and helps to improve digital photography because the photographer can work out what combinations of shutter speed and aperture work well.
Glossary of Digital Photography Terminology – http://www.digital-photography-tips.net