The military or Armed Forces is the branch of the US Government. The different branches have different missions and goals, but all provide paid training and money for further post-secondary education. The U.S. Military has five active-duty service branches and their respective Guard and Reserve units. These offer full time and part time employment immediately after high school or after going through initial training, potentially at a college or university. The military doesn't accept everyone; in order to enlist, you must qualify. There are age, citizenship, physical, education, height/weight, criminal record, medical, and drug history standards.
You may consider the military is you are interested in learning skills in service to the US and don’t mind having your time structured for you. Additionally, you might want to explore the world or want to get money to pay for you college. No matter what, you’ll need to be willing to commit for a significant amount of time as military commitments tend to last 4-8 years.
ROTC or the Reserve Officers' Training Corps offers students a chance to study and serve after they complete their degree, or to participate for a short time in the program without a post-college commitment.
Students train as they learn, taking military science courses alongside regular college classes, with the requirement to enlist as an officer in the Army, Navy, or Air Force upon graduation. The Coast Guard does not have an ROTC program, but rather a College Student Pre-Commisioning Initiative. More info on ROTC here.
Check out the government's website on how to enlist in the military and reasons why you may consider it as an option.
Today's military gives an overview of the different branches and different ways to serve.
Military Info Sheet