This is custom tag .It gives us several benefit like using model attribute,using validation.basically when the page is executed java classes are executed which is defined in tld file. This way we can separate business logic from view layer.
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