A sort of fixed position
I was on the Saints Row website (http://www.saintsrow.com/) the other day and liked the footer thing which appears fixed to the bottom but when scrolled all the way down it reveals a bit more. Also, this site (http://www.wow-professions.com/) has something similar. One of the adverts down the side fixes itself when you scroll down.
Is this done using just HTML & CSS if so, how is it done?