JDK

JRE runs Java programs, but cannot compile Java source code. For that you need a bigger gun.

Enter JDK.

JDK - Java Development Kit - contains the Java compiler and several other development tools. It also contains all the Java standard libraries that Java programmers use in their programs.

Note - JDK is a SuperSet of JRE. It contains JRE + the compiler + more development tools + Java standard libraries

Java developers usually install just the JDK and do not worry about installing a JRE separately - it is already included in the JDK.