even i do search in google, and read some of the article, but i still not clear what portlet in java can do for us (english is not my native language, sometime is hard to totally understand what people wrote)

i was discussing with my friend between java and php.

most of the php forum, they allow user to install forum plug-in on the fly within the forum system. some of the forum system even have a plugin list in the backend, the list is displaying all the plugin offer from that forum system, after clicking install, the system will download all the file need and install into the local forum system. 
just like we purchase iphone app from app store 

while in java, we all know if we want to add any functionality into a web application, we have to recompile build the war again, and deploy onto the server container.
Seems it is not possible to do things like php did.

however,　there is a application JIRA, seems it was writen in java technology, and they allow their user to do things justs like the php application i descrip above.

and i happened to know the term "portlet", both spring and apache have such kind of project as well. Spring Portlet MVC and Apache Jetspeed

so does it real those portlet framework can offer us to build web application like those php forum did???