How search engines identify your site and its contents is a big question ever.We had tried so many techniques to list our sites in the search engines based on some key words.But if somebody adds more words their site will overtake ours. Now the major search engines unite together to have a common schema for the web data. ie as a web developer we can tell the search engine about the nature of our site. The schema contains itemscope and itemprops using which we can specify the category of the data in the site. If our site is to list the movies we can specify our movie name html element as Movie and director name as itemprop director.
This enables the search engines to identify that this data is about a movie and the director name is mentioned.So search engines can store them structurally which help to give better search results.one eg is given below which I took from google blog.
<div itemscope itemtype ="http://schema.org/Movie">
<span>Director: <span itemprop="director">James Cameron</span> (born August 16, 1954)</span>
<span itemprop="genre">Science fiction</span>
<a href="../movies/avatar-theatrical-trailer.html" itemprop="trailer">Trailer</a>
see the new words itemscope,itemtype and itemprop.These are the special schema words to identify the category of the data in our site which helps the search engines to build a good search index .
As I mentioned earlier ,another important aspect is this is a combined effort from the major search engines such as google,yahoo & bing.Clicking on the link will give you what they say about schema.org.