Rollovers are a basic form of user-interface interactivity which visually signal the user that their cursor is over a clickable area. They use javascript to replace one image with another. Sliced images can be hand-made but are generally generated by programs such as Fireworks and ImageReady. You use it with Photoshop images to divide the image into separate pieces and then assignment events and actions to those pieces. The images are placed in tables and can be linked to various javascript functions.
Drop menu are usually selectable list of locations.
However, since layers will not be supported in Netscape 6.0 they are on the way to becoming another historical curiosity. Browser implementation varies and consistent cross-browser positioning requires a branching script. Information received from forms is normally processes by the server with cgi scripts. CGI's may, or not be permitted by your service provider. They can be a security problem so most providers limit their use.