HTML5 Micradata is something that extends the Microformats and addrsses the shortcomings of Micorformats. Schema.org is one of the most prominent Vocabularies inside the HTML5 Microdata, so not on easy to extend RDF and RDFa, but on prepertiery Schemata that were predefined. The Intention is that Search Engines, Web Crawler and Browser are able to recognize and to understand this schema.org Markup.
Microdata annoated te Domain Object Model Tree which is inside the HTML5 representation/encoding of the Web Page with name/value paris that are scoped and come from custom Vocabularies that are predefined. All of this is centered around custom Vocabularies which means veryone can define a custon Vocabulary and start embdedding custom Properties into Web Pages. It works with name Value/Pairs, every Microdata Vocabury defines a set of Named Properties and it applies the Concept of Scoping which means Microdata reuse the hierarchical structure of DOM itself.
The Microdata Vocabularies are refering to specific things like Categories. What they do is that they use global Attributes within HTML Tags that are used especially for Microdata. The Scope creates a Resource,
itemtype means which Vocabulary to use,
itemid identifies the Resource.
itemproperty is a Property for the Resource and
`itemrefer references some other Resources.
Microdata Global Attributes