OpenJDK is Oracle's free, open-source implementation of the Oracle JDK. It is where Oracle does all of its JDK development. You can use it with no restrictions. If you want to pay for support, you can pay Oracle or any of several other companies that package up and support the OpenJDK. If you choose to pay Oracle, you'll get the Oracle JDK which is identical to the OpenJDK except for a couple of error messages and the ID string.