This may not be the official definition but I'll give it a try.
A Form Bean is a JavaBean in the View component of MVC. It contains private variables (usually all in the form of Strings) and public getters and setters. There is also a reset() and validate() as part of
Struts. With Struts, your Form Bean extends ActionForm or ValidatorForm and is wired up by the struts-config.xml and tiles-defs.xml files. You also tie in your
JSP and Action to use your Form Bean to get and set form values.
That's it in a nutshell. I'd pick up a Struts book for a deeper explanation.