Hi,
Well if u are copying the class file in WEB-INF\classes folder and u have checked the option of reloading enabled(u can mention this at the time of deployment) than u dont need to restart the app server again to reload this class.This is true for web modules.
For
ejb module, if u are modiying the jar by replacing the new class file than u have to restart the app server for it to get loaded.
Hot deployment is when u have to only restart the ejb module(for making the changes to reflect) in which u have changed the class file and not the whole appserver.
Regards,
-R