Javascript is the language most often used for functions such as changing images when the cursor moves over a menu item or popping open new browser windows etc.
Although Javascript is well suited to handle these operations, it is a fully fledged programming language capable of implementing far more advanced client side functionality and interactivity.
We use Javascript extensively on most of the web sites we create to create interactive drop down menus and numerous other features designed to maximize the usability and navigation of the site.
Javascript also offers a limited degree of server side functionality although languages such as PHP are better suited to server side applications.