Metadata is information about data. Metadata is a definition or description of data such as attributes and structure. This can refer to the content of the data, but also to technical aspects. For example, for an Ogg Vorbis music file, there could be the following attributes:
- Technical: bitrate, creation date, encoding method, encoder version etc.
- Content: artist, album, genre, recording date etc.
Because technical metadata is derived by machines and can be validated by them, it is much more likely to be accurate than content metadata, which lazy humans are notoriously bad about providing. Information retrieval techniques work around this laziness by extracting meaning from the content of the files themselves or matching similar information to existing metadata. A file's hash can also be used to look up metadata in a metadata catalog.
Metadata can be embedded into files (as is the case with most music and video file formats) or stored separately from them (as is typically done for images). Modern file sharing systems try to extract metadata and make it searchable. There are several open metadata catalogs, for example, Bitzi and FreeDB.
Metadata can also refer to community interaction and include ratings and comments by different users who have evaluated the file. So-called "meta-metadata" can vouch for the accuracy of metadata about a particular file.
Meta-knowledge is knowledge about knowledge, or more accurately, knowledge about what we know, what we is unknown. Meta-knowledge describes the rules which differentiate between the two. This aids human reasoning for a better future.