I have no idea what JROTC entails, but as a former Air Cadet, it's best described as being similar to a Scout group but supported by the military.
For example, as an Air Cadet I learnt about principles of flight, had the opportunity to fly light aircraft and gliders provided by the RAF at Air Experience Flights and Volunteer Gliding Schools, undertook leadership and instructional courses run by the Army and RAF to teach other cadets, learnt fieldcraft, did lots of sports, shot with Lee Enfields rechambered to .22 and the 5.56mm L98A1 (a bolt action version of the L85, lacking flash suppressor and with a different set of iron sights), and basically just played soldiers from the age of 13 to 18.
The websites for the three main cadet groups are here:
http://www.armycadets.com/home/
http://www.raf.mod.uk/aircadets/
http://seacadets.ms-sc.org/
The different services give different levels of support to the cadet organisations, but all recognise that they are an excellent recruiting tool and help to raise awareness of the military among young people, as well as present it in a positive light.