Please summarize and explain Jared Diamond's overall message and theme of his book, Guns, Germs, and Steel?

The general theme of Guns, Germs, and Steel is human development.  Diamond is interested in the different levels of development in different societies.  In other words, he wants to know why some societies became so much more wealthy and powerful than others.  Diamond’s overall message is that these differences are caused by geographic factors, not by any factors that are innate to the people or cultures of the various societies.
In our world today, countries populated by people of European descent are, on average, stronger and richer than others.  This was even more true a few decades ago when Diamond started to think about the issues that he explores in this book.  Many people have attributed this difference to inherent qualities of the people and cultures involved.  They have said that Europeans dominate because they are genetically superior or because their culture is more conducive to progress.  Diamond disagrees.  Diamond says that Europeans came to dominate through geographical luck.  He says that Eurasia was more suited to agriculture than any other region in the world.  Therefore, agriculture arose there first and spread more easily in Eurasia than elsewhere.  Because this area got agriculture first, it also developed civilization and technology first.  It had a longer time in which its civilizations could grow and develop.  Therefore, by modern times, Europeans and their descendants had come to dominate the world.
Diamond argues, then, that differences in human development around the world are caused by geographical luck rather than by any factors that make the people of one society superior to the people of another.