The height of the hidden area initially is set to 0. Then when the "client login" button is clicked, the height of the hidden area is increased gradually (using setInterval) until it reaches a specified height. Clicking it again decreases the height gradually until it is 0.
You could use scripta-thingy as Matt suggests, but if you do decide on including a whole library just for one slidey effect, I recommend you use jQuery instead. Easier to use and far smaller in file size. http://jquery.com