by visiting portlets faq page i could see many number of implementations provided by different vendors. if one wants to learn basic concepts, the things for developing portlets irrespective of any implementation what should he/she learn? if one wants to develop portlets and portals what are the basic things to be learnt. As far i know some tools even provide drag n drop options to create portlets. is it that one has to just through tools documentation??
The best way to start it to download Pluto or JetSpeed and play around with it.
Check out my signature links. I have a variety of free tutorials that show you how to create a simple portlet and deploy it to JetSpeed with nothing more than notepad and a JDK. From there, the tutorials go deeper and deeper into the Portal API, with full CBTs and code samples.