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 your 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.
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.
How to use microdata in Joomla
This is a question easily answered: you do not have to do anything. The 3.3 implementation just hard coded microdata into the code. Not everything is catered for, however, just the most used features, like articles and contact persons. There will probably be further improvements in later Joomla releases, allowing you to configure and switch on / off microdata (I will make sure to update this article by then), but for now just enjoy them. In my blogpost about the introduction of Joomla 3.3 I already showed how articles are now displayed. 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.
Topics not catered for, and Joomla 2.5
Some topics are not catered for yet, like the breadcrumbs module, and also Joomla 2.5 misses out, another reason to upgrade... 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.
I also created a blogpost on how to set up microdata for the breadcrumbs module.
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.
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.
A special case is the picture shown in the search results:
This is a special case of using micro-data, it is a result of connecting your website with your Google+ page. If done correctly, your face will show up (well, until June 2014, Google removed the feature again, now it's just your name listed). To achieve this, first create an account on Google+ and then follow the directions on the Google+ Signup page.
You can also achieve the effect by making sure you mention your website in your Google+ page as a site (custom link) you contributed to, followed by a link from your website (every page) to your Google+ profile as follows:
Replace [profile_url] with your Google+ profile's URL. In my case this would result in this:
You can check whether you have actually done so (both microdata and authorship) in a correct way by using the test-tool in your Webmaster account: www.google.com/webmasters/tools/richsnippets.
Also you can add structured data using extensions. An example is Google Structured Data Markup by Tassos extensions, and there are a lot more. I will add more soon, this blogpost needs urgent updating ;)