The OpenJDK is the source code. Like a master branch where commits are made.
That branch is always moving forward. New features added, new bug or security patched, etc.
Now there comes points where you need to package the OpenJDK source code into a binary.
This packaging process is done by multiple different "vendors".
Now Oracle packages for free a release every 6 months. That package is the Oracle OpenJDK. If another vendor did a package it would be Other Vendor OpenJDK.
Now the idea of back-porting is basically a cherry pick package. Instead of packaging the latest commit. You cherry pick only bug and security commits and apply them to say the release 8 commit. That way, you don't introduce new features, only bug fixes. Then you make sure you didn't break anything compatibility wise.
The idea is that those should be easier to migrate to, since by leaving out new features, you limit potential backward breaking changes.
This cherry picking merge, which can be painful to do, because sometimes you may need to address weird merge conflicts, or even make some changes to the fix since you're trying to apply it to a earlier commit then it was built on. This is what Oracle now charges money for.
Other vendors also offer back-ported releases for money. Azul and RedHat for example.
AdoptOpenJdk is a new community initiative trying to do backported releases for free with volunteer work.
And Amazon Coretto is doing it for free, but hoping that by moving to their JDK package, you'd eventually end up using AWS.
That branch is always moving forward. New features added, new bug or security patched, etc.
Now there comes points where you need to package the OpenJDK source code into a binary.
This packaging process is done by multiple different "vendors".
Now Oracle packages for free a release every 6 months. That package is the Oracle OpenJDK. If another vendor did a package it would be Other Vendor OpenJDK.
Now the idea of back-porting is basically a cherry pick package. Instead of packaging the latest commit. You cherry pick only bug and security commits and apply them to say the release 8 commit. That way, you don't introduce new features, only bug fixes. Then you make sure you didn't break anything compatibility wise.
The idea is that those should be easier to migrate to, since by leaving out new features, you limit potential backward breaking changes.
This cherry picking merge, which can be painful to do, because sometimes you may need to address weird merge conflicts, or even make some changes to the fix since you're trying to apply it to a earlier commit then it was built on. This is what Oracle now charges money for.
Other vendors also offer back-ported releases for money. Azul and RedHat for example.
AdoptOpenJdk is a new community initiative trying to do backported releases for free with volunteer work.
And Amazon Coretto is doing it for free, but hoping that by moving to their JDK package, you'd eventually end up using AWS.