Three nested boxes: the JDK (tools to build) contains the JRE (what's needed to run) which contains the JVM (the engine that executes bytecode).

JDK = JRE + development tools (javac, jar, javadoc, jdb, jshell…)

JRE = JVM + class libraries (java.lang, java.util, …)

What each one is

	JDK	JRE	JVM
Purpose	Develop + run	Run	Execute bytecode
Contains `javac`?	Yes	No	No
Contains class library?	Yes	Yes	No
Platform dependent?	Yes	Yes	Yes

Inside the JVM

Class loader subsystem
Finds .class files, loads them, links (verify → prepare → resolve) and initialises them.
Runtime data areas
Heap (objects), Method Area / Metaspace (class metadata), one Stack per thread (frames with local variables), PC registers, native method stacks.
Execution engine
Interpreter + JIT compiler (C1 quick, C2 optimising) + Garbage Collector.
JNI / native libraries
Bridge to C/C++ code (native methods).

Checking what you have

Java

public class Main {
    public static void main(String[] args) {
        System.out.println("feature version: " + Runtime.version().feature());
        System.out.println("spec version:    " + System.getProperty("java.specification.version"));
        System.out.println("class file ok:   " + (Main.class.getName().equals("Main")));
    }
}

Output

feature version: 21
spec version:    21
class file ok:   true

Terminal

$ java -version          # runtime version
$ javac -version         # compiler version (only in a JDK)
$ echo $JAVA_HOME        # where the JDK is installed

OpenJDK distributions

All major builds come from the same OpenJDK source; they differ in licensing, support length and extras.

Eclipse Temurin · Amazon Corretto · Oracle JDK / Oracle OpenJDK · Azul Zulu · Microsoft Build of OpenJDK · GraalVM

What is the difference between JDK, JRE and JVM?

Is the JVM platform independent?

What is the JIT compiler?