Which version of JRE for JDK 11?

is a JDK version correlated to the Java language version?

Yes. The number of the kit to produce apps on the Java platform, the JDK, is numbered to match the version of Java platform. 8 for 8, 11 for 11, and so on.

If I use JDK 11 can I run applications that use a lesser runtime for example 8? That is JDK 11 with Jre 8

You can use JDK 11 during development for earlier Java if your project is explicitly configured to be compiled and built for the earlier Java only.

Of course that means you cannot use features added in the later Java. But if your project is properly configured, your IDE should prevent such features from being introduced into your codebase.

What version of the jre is required for using jdk11?

There is no more separate JRE distribution. The JRE was a runtime-only environment used to run apps while the JDK was used to develop (and run) apps.

Now we have only JDK releases, as far as I know.

Oracle no longer intends for end-users to be installing a JRE or a JDK. Java Applets in a browser and Java Web Start app delivery are both being phased out, leaving the end-user with no need for a JRE. Java-based apps are expected to bundle their own Java implementation. The only folks consciously installing a JDK will be developers & server-side sysadmins.

Important:

• Understand clearly the nature of the OpenJDK project, as explained in Wikipedia.
  • Know that Oracle has committed to feature-parity between its own branded Oracle JDK and the OpenJDK project. This commitment includes donations of previously-commercial features such as Flight Recorder and Mission Control.
• Read this white paper by Oracle of 2018-03, Java Client Roadmap Update
• Read the white paper Java Is Still Free, authored by key members of the Java community.

Learn about:

• Java Platform Module System
• jlink (JEP 282)
• jpackage (JEP 343)

Here is a flowchart diagram that may help you finding and deciding amongst the various vendors providing a Java 11 implementation.