Microdata is a WHATWGHTML specification used to nest metadata within existing content on web pages.Search engines, web crawlers, and browsers can extract and process Microdata from a web page and use it to provide a richer browsing experience for users. Search engines benefit greatly from direct access to this structured data because it allows search engines to understand the information on web pages and provide more relevant results to users. Microdata uses a supporting vocabulary to describe an item and name-value pairs to assign values to its properties. Microdata is an attempt to provide a simpler way of annotating HTML elements with machine-readable tags than the similar approaches of using RDFa and microformats.
Microdata vocabularies provide the semantics, or meaning of an Item. Web developers can design a custom vocabulary or use vocabularies available on the web. A collection of commonly used markup vocabularies are provided by Schema.org schemas which include: Person, Event, Organization, Product, Review, Review-aggregate, Breadcrumb, Offer, Offer-aggregate. Major search engine operators like Google, Microsoft and Yahoo! rely on this markup to improve search results. For some purposes, an ad-hoc vocabulary is adequate. For others, a vocabulary will need to be designed. Where possible, authors are encouraged to re-use existing vocabularies, as this makes content re-use easier.
In some cases, search engines covering specific regions may provide locally-specific extensions of microdata. For example Yandex, a major search engine in Russia, supports microformats such as hCard (company contact information), hRecipe (food recipe), hReview (market reviews) and hProduct (product data) and provides its own format for definition of the terms and encyclopedic articles. This extension was made in order to get rid of problems with transliteration between the Cyrillic and Latin alphabets. Due to this implementation of additional marking parameters and routine use of Schema's vocabulary  the indexation of information in Russian-language web-pages became considerably more successful.
itemscope – Creates the Item and indicates that descendants of this element contain information about it.
itemtype – A valid URL of a vocabulary that describes the item and its properties context.
itemid – Indicates a unique identifier of the item.
itemprop – Indicates that its containing tag holds the value of the specified item property. The property's name and value context are described by the item's vocabulary. Properties values usually consist of string values, but can also use URLs using the a element and its href attribute, the img element and its src attribute, or other elements that link to or embed external resources.
itemref – Properties that are not descendants of the element with the itemscope attribute can be associated with the item using this attribute. Provides a list of element ids (not itemids) with additional properties elsewhere in the document.
The following HTML5 markup may be found on a typical “About” page containing information about a person:
<section> Hello, my name is John Doe, I am a graduate research assistant at
the University of Dreams.
My friends call me Johnny.
You can visit my homepage at <ahref="http://www.JohnnyD.com">www.JohnnyD.com</a>.
I live at 1234 Peach Drive, Warner Robins, Georgia.</section>
<section itemscope itemtype="http://schema.org/Person">
Hello, my name is
<span itemprop="name">John Doe</span>,
I am a
<span itemprop="jobTitle">graduate research assistant</span>
<span itemprop="affiliation">University of Dreams</span>.
My friends call me
You can visit my homepage at
<section itemprop="address" itemscope itemtype="http://schema.org/PostalAddress">
I live at
<span itemprop="streetAddress">1234 Peach Drive</span>,
<span itemprop="addressLocality">Warner Robins</span>,
The same machine-readable terms can be used not only in HTML5 Microdata, but also in other annotations such as RDFa or JSON-LD in the markup, or in an external RDF file in a serialization such as RDF/XML, Notation3, or Turtle.