At its most basic level, metadata can be defined as "data about data" -- although this phrase is too broad to be of much use. Instead, we should look at metadata as the information that helps data to be discoverable, reusable, and understandable for others.
Metadata describes the content, quality, condition, and other characteristics of data. Metadata is generally standardized, structured information that facilitates functions associated with data, such as:
Descriptive metadata describes the object or data and gives the basic facts: who created it (i.e. authorship), title, keywords, and abstract.
Structural metadata describes the structure of an object including its components and how they are related. It also describes the format, process, and inter-relatedness of objects. It can be used to facilitate navigation, or define the format or sequence of complex objects.
Administrative metadata includes information about the management of the object and may include information about: preservation and rights management, creation date, copyright permissions, required software, provenance (history), and file integrity checks.