A lot of websites have drop down menus etc that appear when you simply "mouse over" them with your cursor, how do you do this with a touch screen like on the iPad? Or is it not possible?
Yes that is correct. Tapping seems to emulate a "mouse-hover". However sometimes this does not work as the actual hover button might also be a link and will open the hyperlink instead of showing a drop-down menu.
Therefore, Apple actually recommends not to use drop-down menus or to use hover objects when developing for the iPad.
I've found that some of them you have to be very fast - if tapping on the menu links to something, you've only got about a second to tap on the menu item you want. Very frustrating, sometimes it takes a few tries.