What is the relationship between RDF, RDFa, Microformats and Microdata

Question

I've done quite a bit of research but I can't understand the exact relationship between the 3. Also, now that W3C has officially recognised RDFa, would you recommend it over Microdata, taking into consideration that it matches Microdata's features and more?

Answer 1

<sarcasm>I can't imagine what you find so confusing</sarcasm> (edit: these tags were invisible before)

Very briefly:

• Microformats A way to use html pages as both a human readable document and machine readable data, without repetition (e.g. sticking CSV in the head element).
• RDF A data model designed for the web. Schemaless, uses URLs to name types and relations.
• RDFa A way to encode (write) RDF in html, following the style of microformats (i.e. minimising repetition). Works by adding a few attributes to html.
• Microdata An alternative to RDFa, using different attributes and a different data model.

Less briefly, RDF came from attempts to make data 'webby'. There was even a plug-in for browsing RDF's precursor MCF (from Apple, curiously). The data model was designed so that you could write what you want without having to pre-arrange how to column names or key meanings with everyone else on the web. RDF was standardly written (serialised) using XML (although other, more pleasant, formats are available).

So in this world you might have a readable homepage (me.html) and a data homepage (me.rdf) for machines to scoop up. Those machines won't understand the meaning of <p>I live in <a href="http://example.com/Chicago">Chicago</a></p>, but they might be able to use :me ex:livesIn <http://example.com/Chicago> if they look up 'livesIn'.

Microformats also tries to make data 'webby', or perhaps that should be the web 'data-y'. The insight here is that there's a lot of data in web pages, between the prose. If you have a few hints a machine can work out that that piece of html above is basically an address. Those hints are microformats. Typically they use conventions around html class names to indicate that the content is more than text.

So microformats don't need a separate web page for machines. But microformats only cover a few types of data (addresses, friend links, position...) because of the way they work. Each is a particular convention which needs to be agreed. Used without care they can mix badly, too.

RDFa is an attempt to get both the flexibility of RDF and the simplicity of microformats. Microdata was a response to RDFa, but with a different data model (roughly arrays and hashes rather than a relational model). Unlike microformats neither dictate what sort of information is provided, just how it is encoded.

To complete what's missing we now have schema.org, which is an initiative from the big search engines (Google, Bing, Yahoo, Yandex) to index web data. schema.org coordinates those meanings so the search engine knows that, say, events have locations. Microdata was originally used by schema.org, but now both microdata and RDFa are supported.

I'd ignore microformats these days and go for schema.org. My personal taste is the RDFa encoding, but either it or microdata should work fine.