The subject of Microdata for Joomla is an advanced topic. Microdata are also sometimes called Rich Snippets or Structured Data. They help to further structure your HTML by setting up subjects with properties in your HTML code (they do not affect visual display). If you use them properly, you may be rewarded by Google, they love you to structure your data, as it makes it easier for them to understand your content. Also, you can get bonus points from Google, like having stars from readers reviews shown in the search results, or a small video thumbnail, the price for webshop products, etc., as in this example for a restaurant:
If you look at the source code of such a page, you will see something like this:
<div itemprop="aggregateRating" itemscope="" itemtype="http://schema.org/AggregateRating">
<i class="star-img stars_3_half" title="3.5 star rating">
<img alt="3.5 star rating" class="offscreen" src="/stars_map.png.">
<meta itemprop="ratingValue" content="3.5">
You can see that properties are assigned to the tags, which helps search engines to understand the code. The syntax is standardized according to schema.org.
It might not immediately boost your rankings, but for your CTR (Click Through Rate) in the search engine rankings it will help. Research showed that users are 30% more likely to click on your result than a similar result without microdata.
Note that there are several conventions to add structured data. The example above is using microdata, where you add markup to your HTML, but you can also use JSON structured data, where the code is added in the head-section of the HTML, like this example for an address (wrapped in a script tag):
"streetAddress": "20341 Whitworth Institute 405 N. Whitworth"
Google really loves this format. More examples on www.schema.org.
How to use microdata in Joomla
Until Joomla 3.3 you needed plugins and overrides to implement them. Joomla 3.2 introduced some support for microdata, but 3.3 actually made the code available to the regular user, though still only or limited purposes. But now you do not have to do anything. The 3.3 implementation simply hard coded microdata into the code. Not everything is catered for, however, just some features, like articles and contact persons. There may be further improvements in later Joomla releases, but things have been quiet on this front for some time. Another example is the code for Contacts:
<dl itemprop="address" itemscope="" itemtype="http://schema.org/PostalAddress">
<dd><span class="contact-street" itemprop="streetAddress">
<dd><span class="contact-suburb" itemprop="addressLocality">
<dd><span class="contact-postcode" itemprop="postalCode">
Postal code here...
As you can see, it is now very important to use every field exactly what it is meant for. I have often used the address field to put in non-address related information, just because of formatting issues, but this should now be avoided.
In case you do not see any microdata showing up in your HTML, make sure that you do not have template overrides that override the article display. You could also check this with your template provider, they may have updates for this. If you are not sure, look at the folder \templates\YOUR_TEMPLATE\html\com_content\article and check if there is a file called default.php.
Finally, also the breadcrums module now supports microdata, giving you breadcrumb paths in the Google SERP pages:
Topics not catered for by Joomla
Some topics are not catered for yet. In the meantime, you can use plugins to implement microdata.
There are quite a few plugins already for adding microdata, some quite generic, some quite specific. An example is Rich Snippets Vote, which adds the microdata format to the output of the voting system. Also a set of overrides can be applied through Microdata for Joomla. Then there are many components that create their own microdata, like Hikashop for properties like price, stock and reviews, or event-data for RS-Events, etcetera.
I also created a blogpost on how to set up microdata for the breadcrumbs module.
A nice plugin to add structured data to your site is the Google Structured data Markup extension by Tassos.
Finally, nothing stops you from creating the correct HTML in your editor. For this purpose, we've got our favourite editor again, JCE, which has a nice add-on available (from the JCE provider) for just this purpose: Micro-data add-on for JCE. Another solution specifically for the JCE editor is J4schema.
You can also use online generators to create the correct format for your HTML. You can choose generators with multiple sorts of structured data. The format that Google seems to like best these days is the JSON-LD format and an excellent generator for that is www.hallanalysis.com/json-ld-generator.
By the way, some more information on microdata can be found in one of my blogposts. Finally, you can check if your microdata have been implemented correctly by using the new structured data testing tool by Google. Expert users might even add structured data to their site via Google Tag manager, see this blogpost: www.moz.com/blog/using-google-tag-manager-to-dynamically-generate-schema-org-json-ld-tags.