The itemid global attribute is the unique, global identifier of an item. itemid attributes can only be specified on elements that have both itemscope and itemtype attributes. Also, the itemid can only be specified on elements with an itemscope attribute whose corresponding itemtype refers to or defines a vocabulary that supports global identifiers.
The global attribute itemref Properties that are not descendants of an element with the itemscope attribute can be associated with the item using an itemref. itemref provides a list of element id's (not itemids) with additional properties elsewhere in the document.
The global attribute itemscope (usually) works along with itemtype to specify that the HTML contained in a block is about a particular item. itemscope creates the Item and defines the scope of the itemtype associated with it. itemtype is a valid URL of a vocabulary (such as schema.org) that describes the item and its properties context. In the examples below the vocabulary used is from schema.org. Every HTML element may have an itemscope attribute specified. An itemscope element that doesn't have an associated itemtype has an itemref.
The global attribute itemtype specifies the URL of the vocabulary that will be used to define itemprop's (item properties) in the data structure. itemscope is used to set the scope of  where in the data structure the vocabulary set by itemtype will be active.
Microdata is a WHATWG HTML specification used to nest metadata within existing content on web pages.[1] 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.