The prefix “meta” is used with strictness in knowledge engineering. In many other contexts, like the Semantic Web or geographic information science, “metadata” simply means data that contains information about a resource. On the Semantic Web, a resource is everything that can be uniquely identified. It may be a book, an idea, a service, or a person. A description of a resource is usually composed of a set of metadata. For instance, a data set may be described by the following set of metadata: name, size, format, abstract, location. In the field of geographical information, a resource may be not only a data set but also a feature, a data series, a model, or a service.
Practically, metadata are data that should support operations about a resource or a set of resources when it is difficult or impossible to perform these operations with the resources only. These difficulties or impossibilities are various. These operations may require input information that is not stored in the resource. For instance, the operation may be a user selecting a data set, and the required input information would be how often the data set is updated. To support user access to a data set, the data provider should thus document a specific metadata for each data set describing its maintenance frequency.
These operations may require input information that is ill-structured in the resource. For instance, finding a street on a city map can rely on a street index associated with the map.
These operations may be about a set of resources and require information that is ill-structured in the set of resources. For instance, buying aerial pictures, the extent of which intersects a specific river, and which are of good quality, is easily done through an index showing location of pictures and a database recording the quality of the pictures.
These operations may be about resources that cannot be handled. For instance, choosing a map on the Internet cannot rely on the maps themselves but on information about them that can be published on the Internet, like the spatial extent, the scale, and an overview.