What is the difference between countries' and country's?

One is plural and the other is not. The Word in its singular form is "country" as in "Angola is a country." If there are more than one, then it is "countries" as in "Angola and Kuwait are countries."

Now, what happens if you want to talk about something belonging to a country? If there is only one country, you say "country's" as in "The President of Angola has pledged his country's support for the resolution." or "Our country's Natural Resources are pretty much limited to sand."

If there is more than one country, you say countries' as in "The governments of China, Canada and the United States have pledged their countries' support for continued pollution." "I'm planning my trip through Italy and Austria because those countries' weather is better than any of the others."