Is there any difference between using one of this technologies?
I'm building now a site using HTML5, and I'm having hard time to decide which one of them to use. I cannot see any difference between them, else the syntax size, which I'm not quite sure is an advantage for the microformats side.
*Edit, May 2015: Times have changed... again. Schema.org seems the way to go, using either microdata (W3C note) or RDFa (W3C recommendation), where the RDFa Lite variant is easiest to learn. Meanwhile recently Microformats released a new version as well, which nobody is paying attention to currently.
Also see the answer to What is the relationship between RDF, RDFa, Microformats and Microdata?
Edit, August 2011: Times have changed. Forget my recommendation below. Just use microdata and forget that the other two exist.
Microformats: the oldest and the simplest of the three. If the existing specs cover your needs (that is, you want to mark up addresses, events, friend links, or another one of the supported data types), then they are a nice and practical choice. The problem is that you cannot make your own microformat if you want to mark up some kind of data that's not supported by the official specs.
RDFa: This one is based on W3C's RDF data model (it's basically a way of embedding RDF data into HTML pages). RDF has been around for a long time and there's a large amount of fancy tools for doing stuff with RDF data (stores, search engines, query languages, graph visualizers and so on). So RDFa takes you into this big existing ecosystem. But this also makes RDFa kind of complicated, and the learning curve is steeper than for the other proposals.
Microdata: This is Ian Hickson's counter-proposal to RDFa. In spirit, it is an extensible version of microformats. It doesn't have the RDF connection and is simpler than RDFa. It's still very new and hasn't seen much adoption yet, so it's a bit early to tell. Update: schema.org really seals the deal here.
My recommendation would be to go with microformats if they cover your need, and RDFa otherwise.
I would use Microdata given that Google, Microsoft and Yahoo have collaborated on Microdata and formalized the schemas at http://schema.org. There aren't many tools out there and some spec bugs they have (in particular with their examples), it's only a matter of time before it has more widespread adoption.
November 2012:
Now that e-commerce schemas from the GoodRelations project have been integrated into schema.org, you have even more reasons to use microdata. (additionally to the ones @cygri has pointed out.)
Until now, GoodRelations and Schema.org were related, but separate vocabularies. With this integration, the bulk of the GoodRelations vocabulary is now available in schema.org.
For those who had been using RDFa to markup GoodRelations, the situation is unchanged. The developer of GoodRelations, Martin Hepp, said that "GoodRelations will remain an independent vocabulary, and usable in RDFa and other RDF syntaxes."
But most of the sites that have been using schema.org have been employing microdata, including e-commerce giants like WalMart, Overstock and eBay. For these sites, the type of e-commerce information that can be marked with microdata has been vastly extended.
So if you are starting from scratch, go with microdata. But If you are already using RDFa, there is no reason to change now.
RDF is just more powerful as it supports things like FOAF.
If you are not up for that complexity, just use Schema.org.
All sorted and oh, Search Engines like it too.
RDFa -> more resources (blank nodes, CURIE etc..)
Microformarts -> simply and popular, minor resources and no support to custom vocabularies
Microdata -> cool itemref resource, very very new...
Microdata is RDFa reborn tougher as seen from support to CURIEs being absent. schema.org addresses only the cross-cutting concern of search. Unless the content relates to domain specific aspects with semantic significance that merits it's own vocab or is simple enough to be covered by microdata, it might be just noise treated as spammy content by good search rank algorithm.
The fundamental difference from webmaster viewpoint is
schema.org + microdata is for content significant in the context of search
microformats is for the simplest content pieces that don't merit syntactic overheads
other vocabs + RDFa is for semantic content to be utilized through parsers with domain-specific significance outside searching context
and all 3 blend well in XHTML too, that's useful from analogy of serving weblog as feed for instance.
Key point here is there are better reasons to opt for microdata than being early bird at adopting it, while being respectful of earlier, more widely successfully deployed means.
来源：https://stackoverflow.com/questions/2986918/microformats-rdf-or-microdata