Microdata is a syntax, nothing more. It doesn’t come with any vocabularies.
(Same is true for RDFa, but this is, at least currently, a tag on its own, as it should be.)
Schema.org is a vocabulary, which can used with Microdata and RDFa (and possibly more syntaxes yet to come).
Microformats is a syntax coupled with vocabularies. Their vocabularies can’t be used with Microdata or RDFa, and Schema.org can’t be used as Microformat.
- When a user has a problem with her use of RDFa and schema.org, she would have to tag her question with microdata, although she doesn’t use any Microdata at all.
- When a user has problem with his use of Microformats, he would have to tag his question with microdoata, although he doesn’t use any Microdata at all.
- When a user has a question about the correct choice/use of Schema.org properties, she would have to tag her question with microdata, although it doesn’t contain any code (as the question is about the semantics/meaning of the vocabulary, not about a specific implementation in a syntax).