Portlet Lifecycle

I know that this is very simple topics but might be most useful and necessary to understand :
Portlet Lifecycle

• init() 
• Initializes the Portlet
•  processAction()
•  process input from a user action.
•  render()
•  Renders the content output.
•  destroy()
•  Cleans up the portlet
• ------------------------------------------------------------------------------------------
•  serveResource()
• Allows a portlet to serve a resource 
• The serveResource() method can be used to implement AJAX
  use cases.
• processEvent()
         Perform events triggered by other portlets. (IPC) 

Apart from this it is also necessary to get basics about JAVA portlet specifications.
you can  get good idea from http://developers.sun.com/portalserver/reference/techart/jsr168/pb_whitepaper.pdf