Consider the following example:
<html> <div class="product"> <!-- product information comes here. --> <div> </html>
The reader of this HTML fragment immediately sees that this is something about product information. A computer program, like a search engine instead, cannot determine that this information is actually about products.
To help search engines, and potentially other software, understand the meta information behind the information, http://www.schema.org describes a shared vocabulary which can be used to add meta data to HTML tags.
Consider the revisited example:
<html> <div class="product" itemscope itemtype="http://schema.org/Product"> <!-- product information comes here. --> <div> </html>
By adding the itemscope attribute, you are telling the information between the div element is about a particular item. The itemtype attribute narrows this item further as a Product. This way, search engine can potentially add this information to an online product catalog making your products easier to find. Other types are available as wel, for example:
I implemented this meta tags for a new webshop I am building. These attributes are part of the HTML5 standard and therefore cannot be used by an XHTML1.1 strict syntax without passing the W3C validation. To use these attributes, just declare your document as HTML5. You can then use the W3C validator to fix any validation errors which may arise. I think this is worth the exercise.
One step further to the semantic web.