What does a Java Virtual Machine do?

The Java Virtual Machine (JVM) serves as an abstraction layer between Java bytecode and the underlying hardware on which applications run. It simulates a real CPU by executing Java bytecode, which is the compiled output of Java programs. This simulation allows Java applications to run on any platform that has a JVM installed, providing the essential feature of "write once, run anywhere." The JVM interprets or compiles bytecode into machine code specific to the host operating system and hardware, effectively mimicking the functionality of a CPU for the execution of Java applications.

By focusing on the interpretation and execution of bytecode rather than directly dealing with hardware, the JVM allows developers to write Java programs that maintain high portability across different systems. This characteristic of simulating a CPU plays a critical role in Java's versatility as a programming language.