JVM is the virtual machine Java code executes on
The Java Virtual machine (JVM) is the virtual machine that run the Java bytecodes. The JVM doesn't understand Java typo, that's why you compile your
*.java files to obtain *.class files that contain the bytecodes understandable by the JVM. It's also the entity that allows Java to be a "portable language" (write once, run anywhere). Indeed there are specific implementations of the JVM for different systems (Windows, Linux, MacOS, see the wikipedia list..), the aim is that with the same bytecodes they all give the same results.
JRE is the environment (standard libraries and JVM) required to run Java applications
JDK is the JRE with developer tools and documentation
OpenJDK is an open-source version of the JDK, unlike the common JDK owned by Oracle
