TWRoO on 08 August 2012
For the .css it will probably be floating right.
To make the drop down box when you hover over is probably javascript (though I know you didn't ask that)
The rest is all done server side with php and mysql databases. The databases will hold all the user names and info and a link to where the avatar pic is uploaded, then php calls in the right details when you log in (or if not logged in gives the "login/sign-up" option)
