JVM: English name (Java Virtual Machine), that is, we are familiar with the Java virtual machine. Understanding xxx.class only this type of file, it is possible to class bytecode instructions file to identify and invoke the operating system API-up operation is completed. So, jvm is capable of cross-platform Java core
JRE: English name (Java Runtime Environment), we call it: Java Runtime Environment. It mainly consists of two parts, jvm standards and achieve some basic Java class libraries. It jvm relative, the extra is part of the Java class libraries.
JDK: English name (Java Development Kit), Java Development Kit. jdk Java development is the core, which integrates jre and some nice gadgets. For example: javac.exe, java.exe, jar.exe like.
Relationship between the three is: layers of nested relationship. JDK> JRE> JVM.