Dinesh on Java

Java Virtual Machine (JVM) is an abstract computing machine. Java Runtime Environment (JRE) is an implementation of the JVM. Java Development Kit (JDK) contains JRE along with various development tools like Java libraries, Java source compilers, Java debuggers, bundling and deployment tools.

JRE (Java Runtime Environment):

JRE = JVM + Java Packages Classes(like util, math, lang, awt,swing etc)+runtime libraries.

Java Runtime Environment contains JVM, class libraries, and other supporting files. It does not contain any development tools such as compiler, debugger, etc. Actually JVM runs the program, and it uses the class libraries, and other supporting files provided in JRE. If you want to run any java program, you need to have JRE installed in the system.

Note : If u just want to run applets (ex: Online games or puzzles), JRE needs to be installed on the machine.
 
The Java Virtual Machine provides a platform-independent way of executing code; programmers can concentrate on writing software, without having to be concerned with how or where it will run.

JDK (Java Development Kit):

Java Developer Kit contains tools needed to develop the Java programs, and JRE to run the programs. The tools include compiler (javac.exe), Java application launcher (java.exe), Appletviewer, etc…

Compiler converts java code into byte code. Java application launcher opens a JRE, loads the class, and invokes its main method.
You need JDK, if at all you want to write your own programs, and to compile the m. For running java programs, JRE is sufficient.
JRE is targeted for execution of Java files i.e. JRE = JVM + Java Packages Classes(like util, math, lang, awt,swing etc)+runtime libraries.

Note: You will need JDK, if at you want to write and compile your own programs. For running java programs, JRE is sufficient.

JDK is mainly targeted for java development. I.e. You can create a Java file (with the help of Java packages), compile a Java file and run a java file

JVM (Java Virtual Machine):
When we compile a Java file, output is not an .exe but it’s a .class file. This .class is known as Java byte code, It has the Java byte codes which can be understood by JVM. Java Virtual Machine interprets the byte code into the machine code and execute it. It is responsible for all the things like garbage collection, array bounds checking, etc… JVM is a cornerstone of the write-once run-anywhere value of Java programs.

Note: JVM is platform dependent but it makes the Java platform independent.

The Java Virtual Machine provides a platform-independent way of executing code; programmers can concentrate on writing software, without having to be concerned with how or where it will run.

JIT (Just-in-time Compiler):
JIT also known as dynamic translation. JIT is the part of the Java Virtual Machine (JVM) that helps to speed up the execution time. Parts of the byte code that have similar functionality at the same time complied by JIT and saves time needed for compilation. It translates the instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.