What's the difference between //node and /descendant::node in xpath?

Question

I use a lot of XPath when locating elements in web pages using Selenium, and have moved away from using node1//node2 towards using node1/descendant::node2 more recently. What's the difference between the two methods? Is one more efficient than the other?

Example XML snippet to demonstrate:

<div id="books">   <table>     <tr><td class="title">Lord of the Rings</td><td class="author">JRR Tolkein</td></tr>     <tr><td class="title">The Hitch-Hikers Guide to the Galaxy</td><td class="author">Douglas Adams</td></tr>   </table> </div> 

So it'd be:

id('books')//td[@class='title']

or:

id('books')/descendant::td[@class='title']

Answer 1

see http://www.w3.org/TR/xpath#path-abbrev

// is just an abbreviation for the descendant:: axis

Edit

To quote:

//para is short for /descendant-or-self::node()/child::para

That is, it refers to all para which are a child of the context node or any node descended from the context node. As far as I can tell that translates into any descendant para of the context node.