Metadata, simply put, is the description of a resource in order for people to be able to find it. It enables material of similar subject matter, authorship, format, or any number of other common characteristics to easily and consistently be brought together.
Once someone has found an object, the metadata present in its description provides contextual information about the object which can further help illuminate the digital object being viewed and can help people make the decision of whether an object meets their information needs.
Metadata is generally used to refer to descriptions of digital objects, although the term is sometimes used to refer to resource description of all types, including library cataloguing.
In order for searching and browsing to be carried out efficiently, metadata must follow standards. These can be local or international in scope. An example of a local standard is the DAI's Resource Type vocabulary - a designation peculiar to the DAI that is used system wide to bring items of similar nature together (Books, Periodicals, Articles, Photographs, etc.). An example of an international standard (ISO 639-1 or ISO 639-2) is the Language field, which uses internationally recognized codes to represent different languages).
Generally, an international standard is preferred as it allows common and reliable searching across and between systems. Often, metadata is "harvested" into large metadata aggregators, which rely on international standards being followed in order to make sense of data coming from many different institutions and sources. There is also a case to be made that following an international standard is less costly overall, since it requires no local documentation or development.
Local standards, on the other hand, can often take substantial time and resources to document and develop. However, they are still used to offer more specific searching or browsing capabilities based on the types of material present in a collection and the target user group. They can also accommodate new additions or revisions more easily, since they are maintained locally and quick decisions can usually be made.
Most digital library collections employ a combination of international and local standards.