Microdata are a group of name-value pairs that extract data for search engines and site crawlers. Each name-value pair is called a property and a group of them is called an item. Most search engines follow schema.org vocabulary for the extraction of microdata.
itemid – A unique, global identifier of an item
itemprop – Adds properties to an item
itemref – Provides a list of element IDs with additional properties
itemtype – Specifies the vocabulary’s URL that will be used to define itemprop
itemscope – Defines the itemtype’s scope that is associated with it