What is a Data Dictionary or “metadata”?

In a Geographic Information System (GIS), metadata, or more simply "data about data", describes the content, quality, history, format, and usability of spatial data.  Metadata is information needed in order to effectively use spatial data in a GIS.  

Metadata answers general spatial data questions for each dataset:
What does the dataset describe?
What features and attributes does the dataset include?
Who produced the dataset?  When?  How?
How often is the dataset updated?
How reliable is the dataset?
What is the maximum scale at which the dataset should be used?
Are there any other constraints or limitations on usage?
What are the projection, coordinate system, and datum of the dataset?