Microdata is a WHATWG HTML 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.

This is an example of microdata for a generic item.

<div itemscope itemtype="http://schema.org/Thing">
    <h1 itemprop="name">This is an item</h1>
    <img itemprop="http://schema.org/image" src="http://image_url/" alt="">
    <meta itemprop="http://schema.org/url" content="http://content_url" />

If you have multiple items on one page, you can wrap them in an item list.

<div itemscope itemtype="http://schema.org/ItemList">
    <div itemprop="itemListElement" itemscope itemtype="http://schema.org/Thing">
        ...Other nodes go here....

Microdata schema reuses existing HTML node such as div, span, h1, p to define meta property. For additional property, it can be defined by a meta tag. "Thing" is the generic schema and there are a lot more schemas which are the sub-type of "Things". For example, Action, Article, CreativeWork, Event, Intangible, Organization, Person, Place, Product. All schema can be found at http://schema.org.

View the source of this page, and you can take a look at an example of how an article microdata is defined.

To test your microdata schema, you can use this tool from Google.

If you are not sure where to start, try the helper tool here.