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As of Java 7 openjdk is the basis for the official Oracle release of Java, and constitutes the reference implementation of Java SE. So if you've used any recent JDK release then you've used openjdk.

I think there are still some proprietary closed-source bits in the Oracle Java SE releases though.

I'm no expert, but I thought I'd read something to this effect when looking this up recently. (Just in the past couple days.)

It doesn't. OpenJDK provides no binaries for Windows as far as I'm aware. The build setup is pretty complex and requires a combination of Cygwin and Microsoft tools: http://hg.openjdk.java.net/jdk7/build/raw-file/tip/README-bu...

Basically, Oracle's JDK/JRE is the OpenJDK build for Windows that normal people can use. That's it.

Is the use of OpenJDK along with any Java code free for commercial use?.. Or do companies that use OpenJDK have a chance to have Oracle charge them for using its technologies?..

Using OpenJDK is completely free, as far as I'm aware. Redistributing OpenJDK bits is covered by GPL2 with some Sun/Oracle amendments: http://openjdk.java.net/legal/

You might want to read over Oracle's OpenJDK FAQ: http://openjdk.java.net/faq/