JDK vs JRE
JDK stands for the Java Developer Kit whereas JRE stands for the Java Runtime Environment.
JRE is what is needed to run Java applications. Bundled within is an implementation of the Java Virtual Machine (JVM).
JDK includes JRE and many other Java application development tools. For example, the Java compiler (javac) is included only in JDK.
If you want to run Java software, you may simply live with JRE. The JRE installation file size is also smaller.
If you are a Java software developer, you need to use JDK. As said, if you download and install JDK, you need not install JRE since JRE is included within JDK.