It's data about data. More specifically, metadata is a vocabulary that consists of elements or tags that are used to describe a variety of materials including: books, web pages, journal articles, data sets, images, digital exhibitions, and physical objects.
Examples of everyday metadata:
Omeka uses the Dublin Core metadata standard. See the box at right for core elements.
There are many potential elements that can be used in Dublin Core. Here are the 15 basic elements that we as an institution suggest you use whenever possible: