Active duty military service means that you would be committed to serving full-time in the armed forces for a set amount of time, you may live on a military base, and can be deployed at any moment to serve the country. Since individual contribution is higher through active duty than in the National Guard or Reserve, those who serve through this option are also eligible to receive full payment, leave, and other benefits which National Guard and Reserve members may not see.
As a Reserve service member, you would be trained and qualified to be available for active duty when needed. Each military branch has a Reserve and service members typically work for the military and are paid for doing training drills one weekend per month and serving two weeks per year. Serving through the Reserve also means that you may be called upon in times of war, in a national emergency, or during national security threats.
As a National Guard service member, you would work for the state and be tasked by the governor to assist with local emergencies. You may also be deployed in times of war, help overseas, or here at home in the United States. The National Guard is made up of either the Army National Guard or the Air National Guard (through the Air Force). You will have to work and participate in training drills one weekend a month and two weeks per year, but will also receive payment for your service.