Technically, the # has nothing to do with it. It's just used as a filler for the href="" attribute. The actual pop-up is initiated with javascript.
Code:
<a href="#" onclick="window.open('page.html', 'newWin','menubar=no,toolbar=no,location=no,directories=no,status=no,scrollbars=no,resizable=no,width=250,height=250'); return false;">Text to click on</a>
That will open "page.html" in a new window named "newWin", unable to be resized with everything turned off and a width and height of 250 pixels. You can turn window attributes on and off as you desire, it's pretty self explanatory.
In this case I'd add "return false;" so if you've scrolled down the page a bit it won't shoot you back to the top.