Hyperdata -- indicates data linked to other data in other places. This forms a web of data, evolving from the "data on the Web" that is not inter-related (or at least, not linked).
Like metadata, hyperdata express semantics. Metadata indicates local facts of data on either the conceptual essence (e.g., Web 2.0-style tags) or the semantics of display (e.g., XHTML tags). These facts are locally consistent, and are immutable against the variation of the external links. In contrast, hyperdata expresses semantics of local data in relation with other data. Effectively, hyperdata indicates where a local data item sits within a broader network of data items.
A Hyperdata Browser (also called a Semantic Web browser), is a browser used to navigate the Semantic Web. Semantic Web architecture does not necessarily involve typical formats like HTML, which HTML browsers rely on. A Hyperdata Browser specifically requests RDF data from Web servers, often through content negotiation or "conneg" starting from the same URL as the traditional Web browser; the Web server may immediately return the requested RDF, or it may deliver a redirection to a new URI where the RDF may actually be found. The RDF data will generally describe the resource represented by the originally requested URI. The Hyperdata browser then renders the information received as an HTML page that contains hyperlinks for users to navigate to indicated resources.