The heart of the difference is whether the Java API can be copyrighted or not. Google has been using it in the belief that it was not protected, particularly for Android, Google's operating system for cell phones and tablets. Oracle which acquired the software from Sun Microsystems two years ago, has much to gain if it wins rumored to be as much as a billion dollars.
Oracle's case is that its API is copyrighted along with the underlying software code. Google argues that it can't be copyrighted because the elements of an API are like words, in free and common use.
If it loses, Google would have to eliminate the API or pay Oracle's price (or find a substitute to run the small free apps that are already widely used in Android gadgets).
The case took an odd turn last week, when the judge instructed the jury hearing the suit to assume that the copyright does cover the API but that he would issue his final ruling later in the proceedings. I suspect the jurors will take this to mean the judge has already intervened and made their efforts irrelevant. But that possibility will provide Google a basis to appeal.
Then on Friday, the judge sent aspects of the case to the jury link here. The best current status of the suit is described there in detail. For all the twists and turns, go to Groklaw link here for a daily account in extenso of the arguments.
Then today, a European court ruled in another case that an API is not copyrightable link here. Good for it, blocking the steady expansion of the reach of copyright.