what jdk is used for?

Answer 1

Java development kit(JDK) is for Java applications development purpose.
Java runtime environment(JRE) is for running Java application.
if you just want to run Java applications JRE is enough. for development we need JDK.
you can install JRE alone. or if you install JDK, JRE will comes with it

Answer 2

JDK is used for compiling, documenting and running your Java programs. it contains the necessary tools that are required for using Java on your system.