An instance variable is a variable that is related to a single instance of a class. Each time an instance of a class is created, the system creates one copy of the instance variables related to that class.
An instance variable is declared inside a class, but not within a method.

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Java is a programming language which is primarily for producing code which can be used in nearly all computer environments. The language is designed to allow programmers to write the code once and it will work in any Java Virtual Machine.

The Java Virtual Machine (JVM) is a collection of software programs and data modules that forms part of the Java and Java 2 platforms. JVM works across all platforms and it alters Java bytecode into machine understandable code and executes it.

The JDK is the Java Development Kit.
The JDK consists of the Java compiler and related tools which enable users to create applications in Java.
Sun distributes the JDK as part of J2EE, J2SE, and J2ME.

The JRE is the Java Runtime Environment.
The JRE consists of the Java Virtual Machine, the Java libraries, and all other components necessary to run Java applications and applets.
Sun distributes the JRE seperately, or as part of J2EE, J2SE, and J2ME.

A jar file is a Java Archive.
A jar file is a collection of class files.
The jar file standard is based upon PKZip. In fact, you can open .jar files with a normal unzip program!
A jar archive should contain a file called Manifest.mf which tells the Java Virtual Machine which class file contains the main method.

A Java servlet is a Java program which is run on a web server.
A Java servlet is much like a Java applet, except that servlets are run on the web server instead of being downloaded to the web client.

J2SE is the Java 2 Standard Edition.
J2SE features:

Portability
JDBC API for database access
CORBA technology
Java secuity

J2SE consists of two components: Core Java and Desktop Java.
Core Java provides back-end functionality, while Desktop Java provides GUI (Graphical User Interface) functionality.

J2EE is Java 2 Enterprise Edition.
J2EE is Sun’s preferred Java platform for multi-tier enterprise applications.
J2EE enhances J2SE with:

Enterprise JavaBeans components
Java Servlets API
JavaServer Pages
XML

For more information on J2EE, visit J2EE.