what is the # symbol in the url

Question

I went to some photo sharing site, so when I click the photo, it direct me to a url like

www.example.com/photoshare.php?photoid=1234445

. and when I click the other photo in this page the url become

www.example.com/photoshare.php?photoid=1234445#3338901

and if I click other photos in the same page, the only the number behind # changes. Same as the pretty photo like

www.example.com/photoshare.php?album=holiday#!prettyPhoto[gallery2]/2/

.I assume they used ajax because the whole page seems not loaded, but the url is changed.