JDK is a software development program provided by sun Microsystems. Java Development Kit or JDK comes in various version and can be downloaded free from the sun Microsystems. JVM compiler, debugger and other tools are used with JDK for developing java based application & java applets. So make sure that your JVM compiler & JDK versions are same.
For more information on JDK visit to :http://www.roseindia.net/java/beginners/download_jdk.shtml