JDK: JDK (Java development kit) is a Java language development kit, including the Java Runtime environment (Java Runtime Envirnment), some Java tools (javac/JDB/Java), as well as some basic Java class library,
2. The jre: jre is the Java runtime environment, if just want to run Java program you can simply download the jre,
3. The JVM: the JVM (Java virtual machine), is a Java virtual machine, realize cross-platform, through this virtual machine, can regard it as a miniature of the operating system, and can perform on Java bytecode program, and the operating system is on the software instead of hardware implement,

The differences and relations among: the JDK includes the jre, and JVM, jre inside and contains the JVM, JDK also has a series of development tools, so that is the JDK is equivalent to a development and operation environment, and the jre is a running environment,
In the JDK directory has a bin and lib directory, bin is equivalent to the JVM, and lib is the JVM in explaining the class files needed to mobilize libraries, namely the JVM + lib=jre,
Write a Java program of the overall process is, in the midst of the JDK developed after a Java program by javac to write in the JDK into Java bytecode, on the jre to run these bytecode, at the same time with the JVM parses the bytecode, which is mapped to the CPU instruction set or OS system calls,

4. Java cross-platform principle, explained the function of the JVM in front, is the Java program interpreted as byte code, can be compiled into such programming language, suitable for different platform to achieve the realization of the cross-platform, which means to realize cross-platform Java applications, the corresponding platform to install the JVM, in this process, the JVM is just as a compiler, will not affect the Java program itself, also became a "write once, run anywhere",

5. Java language, Java is developed by sun Microsystems in 1995, the development of the basic aim was to create software can be embedded into the consumer electronics equipment, build a language that is portable and can be cross-platform, James gosling and his other team members to develop, the Java language is both compact and security, its naming sources meeting because at that time the team name when drinking coffee from the island of Java, so simply called Java,
In 1996 launched the first version of the JDK, launched in 1998, after the jdk1.2, this is a milestone version, later renamed java2 for distinction, namely the second generation of Java, and the Java into javaSE (standard), javaME (miniature version) and javaEE (enterprise edition), and subsequent versions because jdk1.5 changes were relatively, so named jdk5.0, henceforth is jdk6.0, such as 7.0,