title: JDK/JRE/JVM/Java SDK | What do they all mean? Sometimes you can develop with JRE and sometimes you need JDK?
body: <p>To tell the truth, I am quite confused on all these terms (JDK/JRE/Java SDK). I am not sure what each one does.</p>
  
  <p>When I first started doing simple java examples in eclipse, I am pretty sure I only had the JRE, which I believed was the default java installer regular users use to be able to run java programs/applets on their system.</p>
  
  <p>However, now in class we are using Google Appengine, and this requires the JDK which I am guessing is the same as Java SDK. After wasting some time finding out that installing the JDK meant I also had to add java/bin to the environment variables to get javac -version to work in the command prompt I find that only the JDK has javac...</p>
  
  <p>How were my early java programs working without having installed the JDK and therefore not having javac? And really the main question... What is the difference between the JRE and JDK, and when do you use each one?</p>
  
  <p>Thank you :)</p>