The first agricultural revolution was 11,000 years ago when we started domesticating plants and animals. It changed us because it allowed us to eat certain crops we weren't able to before, such as corn.
The second agricultural revolution was based in Western Europe. It was about industrializing. New machines were being made, it was an improvement for farming that required less labor. It was from the years 1700 to 1900.
The third agricultural revolution started in the late 1900's and is still continuing today. It is about the improvement of technology and mechanization. Reduced amount of labor. This revolution is known as the green revolution.
Genetically modified organisms are plants or animals that had the cells changed to improve farming. They are good because they can make farming easier and more profitable. The downside of them though is that they are somewhat risky towards bacteria.