There were 15 dublin core elements in the dc: namespace. All of these elements were optional and repeatable. DSpace, OAI-PMH, ... were based on these elements.
contributor, coverage, creator, date, description, format, identifier, language, publisher, relation, rights, source, subject, title, type
The existing 15 elements were extended with range and domain and new elements were added. To ensure "backwards compatibility" with existing implementations, these new changes were located in a new namespace, dcterms:
So dc:creator can't be confused with dcterms:creator, a property that can now have a range and domain.
If other new standards or applications will require the use of the dcterms: namespace instead of the dc: namespace, DSpace won't be able to comply.
Range defines the vocabulary or syntax which must be followed by a property. For example, the coverage property has following range:
http://purl.org/dc/terms/LocationPeriodOrJurisdiction
Examples: Format has the range MediaTypeorExtent and it's recommended to use the MIME types there.
If a dcterm property specifies a domain, it means that the property can only be applied to a specific type of item it describes.
Example: The new term BibliographicCitation has the domain Bibliographic resource. This means that by specifying the property BibliographicCitation, it automatically implies that the item you're applying it to is a bibliographic resource.