Maybe when you first study these things, it's hard to discern what is being done by CSS, JavaScript, or a server side language like PHP.
As a general rule, most of the little effects you see that involve movement of the mouse to get something to change or appear take place in the browers and will be done with JavaScript. That would include rollover effects, drop menus appearing, little pop-up windows.
If you want to actually process something or send something to your web server's database, then that will be done with a langugage like PHP.
Do a google search for "javascript" and you'll be overwhelmed with the websites that provide free snippets. After you browse through some of their directories, you will have a better idea of what can be handled by JavaScript and where to turn your attention if you are interesting in a particular effect.