This is complete collection of Java stuff, as it has compiler which convert source code (*.java) to Java bytecode (*.class), jar acrhive to package class library to jar file, VisualVM to let user visualize the performance and memory capability… and many more.
JRE consists of the Java Virtual Machine (JVM), Java platform core classes, and supporting Java platform libraries. JRE is the runtime portion of Java software, which is all you need to run you Java application for eg: applet in web browser.
When you run your Java application as “java
JIT use to compile bytecode into machine specific native code, to optimize the targeted CPU and operating System. JIT is part of Java Virtual Machine.