Hi,
Unlike HTML's <form> tag which is quite simple to understand i guess <form:form> is a custom tag and quite intelligent too.
A Custom Tag is something which has a proper java code written behind it so that it can provide some useful and specific functionality.
Behind every tag there's a tag handler where the code for that tag is written. When your
JSP (at runtime) is compiled into a
servlet these tags become active and process the code written for them in the tag handler. You can create a tag of your own too ... take a look here
Creating Simple Tag Handler
cheers