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There has been a lot of confusion from a wide varying uses use of this term for various different historical reasons, leading to uses which are sometimes ambiguous and in places inconsistent. This article attempts to shed light on the issue.
Historically, URIs were used to point to thinks like web pages and files and movies, on the web, useful documents, or "online resources" in the sense of useful things out there. FTP. Gopher and HTTP sites served up various types of online resources. People got used to http://example.com/ being a web page and http://example.com/#contact being an anchor within it.
The Online Information community, into whose domain the web stuff was put for standardization at the IETF, referred to these things like web pages as resources, and changed the original "D" for "Document" in "UDI" to "R". Some felt that resource was more appropriate term, maybe because "document" wasn't wide enough to include things like movies.
Now the URI spec actually allowed URIs for completely different things, such as telephone end points, and wisely the URI spec does not make any arbitrary constraint on what a resource should be, especially a resource denoted by a URI in a new scheme to be invented.
Meanwhile, the HTTP spec was polished and elaborated basically as a document delivery system, plus other methods for updating documents, plus POST. (POST started historically as a way of introducing a new web page y posting it to a list, just as in NNTP. It then almost immediately got used as a catch-all extension method. I will ignore it in this overview).
There was no real definition of what a resource or document was -- maybe because it seemed obvious. The HTTP spec did not even specify whether the URI denoted a person or a document about them, it just explained that the thing returned representation of the resource.
Roy's REST work then came along to formalize HTTP as REST and declared that a resource was a time-varying mapping between URI and representation. That was good enough for HTTP. It didn't have enough for the AWWW, when it came along, to be able to describe how the web worked.
In fact, the AWWW document, to explain how to use the web properly, had to add in a bunch of stuff about the social expectations -- things like, yes, the mapping from URI to representation is a function of time, but not just any old one -- a random function is not typically very useful. There are expectations about it can change with time. Persistence, consistency, with various common patterns which allow the web to be a useful medium. The AWWW decided to use the term "Information Resource" for a thing like a web page which contains information, and "Resource" for any old thing at all.
So HTTP and the REST work of was done very much in this space of document delivery, editing and update. There was no philosophical need to talk about what he URI denoted (the person, the web page about the person) until RDF came along, when there was an immediate need.
When RDF was first developed, it was motivated by the need for data about resources very much in the online information sense: data about documents, or 'metadata'. In fact it was designed to be able to describe anything, but many early users of RDF referred to it as metadata technology. RDF used the word "resource" rather awkwardly in fact as it turned out. In the beginning, many of the things being described were documents, and so the online information meaning of resource made sense. But in fact in RDF the resource was allowed to be anything at all. A class, rdf:Resource even used the term as the universal class of all things. A little later, the Web Ontology Language decided to use Thing for that.
RDF came along in what I think was a neat way. It used completely existing web protocol extension devices to introduce a new system which was fundamentally different from the old HTTP+HTML one. The HTML web was a hypertext model, which pages and anchors. The RDF model was a knowledge representation one of arbitrary things. It did this by using the fact that a new language can define whatever it likes as what a local identifier denotes. A graphic language might use local identifier to denote lines and points. HTML used local identifiers to identify hypertext anchors. RDF used them to identify arbitrary concepts, people, whatever.
The web architecture gave all these languages a common way of building a global identifier for the thing denoted by a local identifier in a given document. The semantics of the hash sign are defined web-wide to mean that "a#b" can be used to denote whatever is denoted by "b" in the document denoted by "a".
Worked a treat. At the beginning of the century, people played around and gave all kinds of things URIs like "http://example.com/ foo.rdf#color". Some of us did lots of work and made all kinds of systems which exchanged and integrated data in this way.
Two snags occurred, as the years passed. One was that a bunch of RDF users got the fact that it was good to use HTTP URIs, but didn't get the fact that you should put the foo.rdf online so that people can look up what #color means in it. And as they didn't do that, they didn't actually bother with the "#" at all. The second fly in the ointment was that some people wanting to use RDF for large systems found that they didn't want to use the "#". This was sometimes because the number of things defined in the same file was too low (like 1) or too large (like a million) and it was difficult to divide up the information into middle-sized chunks. Or they just didn't like the "#" because it looks weird. But for one reason or another people demanded the right to be able to use http://example.net/people/Pat to denote Pat rather than a web page about Pat.
This potentially led to huge failures in the whole RDF world, with systems already built which just used "http://example.net/people/ Pat" to identify the document whether you like it or not. I among others pushed back against using non-hash URIs for arbitrary things his but eventually gave in.
So in response to this, the HTTP protocol was, in fact, changed.
The spec wasn't changed. The spec editors were not brought on board to the new model. The spec was interpreted. The TAG negotiated in a way a truce between the existing HTTP spec, RDF systems, and people who wanted to use HTTP URIs without "#" to identify people. That truce was HTTPRange-14, which said that yoiu don't a priori know that a hashless HTTP URI denoted a document, but if the server responded with a 200 then you did, and you had a representation of the document. If you did a get on one of these new URIs which identified things were not documents (people, RDF properties, classes, etc) them the server must not return 200, it can return 303 pointing to a document which explains more.
So the HTTP protocol was, effectively, changed. The HTTP protocol as extended now allows HTTP to be used not only for Documents but for arbitrary Things. It extends the set of things which you can ask a web server about from documents to anything. It isn't a very bad design, nor very beautiful. Other designs would have worked, but that one was the only one which didn't have major problems for some community. It could be extended, but basically it works. It would be very expensive to reverse it in terms of systems which have been deployed.
It is also very expensive to go on debating it as though it is an open issue. It is reasonable to try to make the documents more consistent.
Anyway, that is a simplified version of the history of all this as I saw it.
I would like to see what the documents all look like if edited to use the words Document and Thing, and eliminate Resource. That's my best bet as to two english words which mean as close as we can get to what we want. Note however that the web is a new system, a design in which new concepts are created, so we can't expect english words to exist to capture exactly the concepts. So we take those nearby and abuse them as little as we can as far as we can tell at the time, and then write them in initial caps to recognize that that is what we have done
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