"C" schools are the Navy's Advanced Training schools. "A" schools are considered Basic Operator or Basic Training for a particular rating. "C" schools go further in that they go above and beyond the basic rate training given in A schools.
For example, as a Submarine Sonar Technician, I went to Basic Sonar Operator school first, or Sonar "A" school. After that, I spent another year in an advanced maintenance course (C school) for a specific Sonar system. For other rates, such as Machinists, it can be advanced training for multiple systems, or just one specific system, depending on their area of expertise.
Those who wish to get C school training typically are required to give the Navy at least 6 years due to the length of time it takes to train people. From Boot Camp to the time I reported to my first boat, it took about 18 months of training.
The difference is that you gain more professional training in your career path, which will help you whether or not you stay in the Navy or if you get out. I was fortunate in that I went to 2 C schools, and was trained in both analog and digital electronics systems. That background gave me a pretty good post-Navy career.
