The main features of Java include Object-Oriented, Platform Independence, Security, Robustness, Multithreading, and Performance.

The Java Virtual Machine (JVM) is a runtime environment that allows Java bytecode to be executed on any platform. It acts as an interpreter and provides platform independence by converting bytecode into machine-specific code while managing system resources and memory.

JDK (Java Development Kit) is a software development environment used for developing Java applications and includes the JRE and development tools. JRE (Java Runtime Environment) includes the JVM and libraries necessary to run Java applications. JVM (Java Virtual Machine) is the runtime environment that executes Java bytecode.

The 'final' keyword in Java is used to define constants, prevent inheritance, and restrict method overriding. A final variable's value cannot be changed once assigned, a final method cannot be overridden, and a final class cannot be inherited.

Garbage collection in Java is the process of automatically freeing memory by deleting unused objects. It helps in managing memory efficiently by ensuring that memory is reclaimed and reused, thus preventing memory leaks. The garbage collector automatically identifies and removes objects that are no longer referenced.

The '== operator' is used to compare the memory addresses of objects, meaning it checks if both references point to the same object. The 'equals() method' is used to compare the actual content or state of two objects. For example, in the String class, 'equals()' compares the sequence of characters within the objects.