Metadata is often referred to as "data about data". It serves to describe the contents of a dataset in general terms - it is effectively catalogue information (a typical library card catalogue, or searchable database, is a classic example of metadata).
There are many different metadata standards. Some of the most common are:
Dublin Core - developed by the library and archiving community
Federal Geographic Data Committee - developed by the US government for geographic data
GILS - Government Information Locator Service - developed by the US government for administrative use
ECAI has decided to adopt the Dublin Core standard.