Well if you have come to this website then its pretty certain that either you or your company has decided to take a plunge into the Portal world. And trust me the plunge will certainly pay off.
To get started lets understand a little about the Portals and the Portlets.
When you see yahoo.com or igoogle.com (Portals)then you see a webpage that is composed of several boxes(Portlets) and which can be removed, minimized, customized and so on.
Well in short here Portal is the webpage we are talking about and Portlets are the applications running within that. So a Portlet is basically a web application that can be made part of the Portal. Generally when we talk of Portlets we refer to JSR 168 or JSR 286 compliant Portlets. These are specifications which help you develop Portlets which are supported on most of the available Portal Servers available.
Now what is Portal Server. If you have worked with Servlets then understanding Portlets will be quite easy. But even if you have not do not worry. As we said Portlets are web applications that are developed using JSR Specifications. But any application needs an environment to execute.
A Portal Server basically provides this environment. Like a Servlet Container supports the life cycle of a Servlet, a Portlet Container supports the lifecycle of a Portlet.
You can find a number of Portal Servers available, both open source like Liferay, Apache and JBOSS and then those which need license like IBM Portal Server and BEA WebLogic portal.
To get started with any technology is to know about the specifications governing them. JSR 168 outlines the Portlet development life cycle and various classes and interfaces available.
For your convenience I have provided a document having the important concepts from JSR specification provided by SUN. I still recommend going through the complete specification at the following link:
Complete jsr 168 Specification available from SUN
JSR 168 Specification in a nutshell or in summarized form is attached. Here I have mentioned the important points from the JSR 168 document so that you can just go through this if you are running short of time.