jQuery data() method.

Many times we need to add some data to DOM elements. Like if you want to hide particular div on click of a button, In such cases I used to use class of button to store id of div which we want to hide. Or may be we want to keep a counter of how many times a button is clicked.

Using id or class or JavaScript variable serves the purpose but many times it gets bit complicated. jQuery data() methods provides a cleaner approach to associate arbitrary data to DOM elements.  
Method Signature:

.data(key,value);

key : A string naming the piece of data to set.

value : The new data value; it can be any Javascript type including Array or Object.   

In given jsFiddle, using data method some value is stored for span with id="foo"

For more details : http://api.jquery.com/data/