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

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.

There's a difference in the context group. //para[1] is short for /descendant-or-self::node()/child::para[1], which returns every para that is the first child of its parent. /descendant::para[1] returns only the first para in the entire subtree.

In your case

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

and:

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

return the same result.

But in fact, like it was already stated before, id('books')//td[@class='title'] means id('books')/descendant-or-self::node()/td[@class='title'] which is different from id('books')/descendant::td[@class='title'] in concept.

See the following note:

NOTE: The location path //para[1] does not mean the same as the location path /descendant::para[1]. The latter selects the first descendant para element; the former selects all descendant para elements that are the first para children of their parents.

this note was taken from http://www.w3.org/TR/xpath#path-abbrev