The above code is a self contained HTML doc. The script is where we keep all our functions. language attribute tells the browser about the lang the functions are written in. Similarly we have scripts run at server with runat="server" attribute.
The rest in the script is simple javascript and is very similar to C/C++. We define a func, which is called when the mouse is taken over the part of the web form, for whom we have given a unique name. On this it changes the source of the image via the javascript function.