Originally posted by rathi ji:
expecting a good answer ,
Well, since this is all available on Sun's site in great detail, you're going to get a quick and dirty answer from me. Also, please note that as an American, your tone sounds demanding and impatient, but I freely admit this could easily be a difference of language. As you're posting a lot to these forums lately, you might find answers more forthcoming if you make your tone more friendly and humble. That's my two cents.
The JDK is the Java Deelopment Kit. It allows you to compile Java code and do other development tasks (work with JNI and RMI, debug, etc).
The JRE is the Java Runtime Environment. It allows you to run Java programs.
You can install the JRE by itself (useful for client machines that only need to run code), but the JDK includes the JRE for obvious reasons.