what do <form action="#"> and <form method="post" action="#> do?

I'm reading a book on html development (which I'm fairly new at) and despite the fact that the book just had its 1st publishing one month ago (Nov. 2011), the author is an experienced coder and maybe using "#" for the action in a form is old school?

Because I'm trying to get the gist of the sample code and I cannot find an explanation of form action="#" despite searching for

   <form action="#">   

on google, on SO, and in www.w3schools.com.

Anyone know what the "#" action means for forms?