New to Java

Hi Friends,
I am goin thru a servlets+jsp application..tryin to understand it.In the application,the jsp pages are not displayed directly i.e with a *.jsp extension....instead a *.do extension is used.The *.do extension is an action which is passed to a central servlet, which further redirects it to the corresponding jsp.So,though the browser url displays a *.do extension,the contents displayed are from a jsp page.I am pretty confused with this *.do extension thing,tried searchin on net..but couldn't find much information.I would be obliged if anyone of you could throw some light on this or redirect me to some useful link.
Thanks
Mic