(From digitalcameraworld.com)

What is metadata?

Don’t worry, you’re not alone. It confuses many photographers at first.

The metadata definition generally used is that metadata is simply a set of data that describes and gives information about other data. For photographers, that ‘other data’ is your images. Your digital camera will embed information in each photo it takes that identifies what camera created the file, the exposure information and more.

Read the full article here:
http://www.digitalcameraworld.com/2012/02/28/what-is-metadata-copyright-photos-in-4-steps/