It is simple to say that JDK is the SDK used by developers, which provides Java development environment and operating environment. SDK is Software Development Kit Generally refers to a software development package that can include a library, a compiler, and the like. JDK is Java Development Kit Jre is Java Runtime Enviroment refers to the Java's running environment, is a user-oriented Java program, not a developer.
Everyone may be familiar with Flash, if you design a Flash program with flash, this tool is obviously SDK. If you use IE to add Flash plug-ins, IE and plugins are similar to JRE.
