The cadet forces are military training groups, typically attached to
secondary schools in
Britain. Members are aged 13-18, wear
military uniform while on duty (usually two evenings a week and weekends, plus a week at Easter and 2 weeks during the summer), are attached to
regular army or
air force regiments, and trained in a variety of the
British Army's tactical operations. There is no obligation to join the regular forces afterwards, but many do.
There are three main branches, the Army Cadet Force, the Sea Cadets and the Air Training Corps. I was a member of the 4th Cadet Regiment, Royal Artillery, my brother was a member of 2203 Squadron, ATC. (As a side note, the reason that in Britain we have a Royal Navy and a Royal Air Force, but not a Royal Army is that individual Army regiments are granted Royal status or not, for example the Royal Artillery, Royal Armoured Corps, Royal Signals, etc). The Combined Cadet Force has the advantage that it offers activities from all three services, but the disadvantage that it doesn't have a relationship with any particular unit, and cannot train in such depth or with access to facilities. Cadets learn drill, fieldcraft, skill at arms, first aid, navigation and military history. ATC Cadets take flying lessons and study basic aeronautical engineering, but don't do the weapons or tactics training that ACF Cadets do (I have no experience of what the SC does, but I presume it involves warships).
The Cadet's basic weapon is the L98A1 which is a stripped-down SA80 without automatic fire and SUSAT, but Army cadets are trained on the Browning Hi-Power (a 9mm semiautomatic pistol), SA80 (5.56mm assault rifle), Light Support Weapon (an SA80 with a longer barrel and a bipod), General Purpose Machine Gun (7.62mm, belt fed), LAW-80 (a light anti-tank weapon), Blowpipe (shoulder mounted SAM) and Light Gun (a 105mm towed artillery piece). Army Cadets are taught to patrol forests, assault buildings and defend bridges. Cadets have ceremonial duties, for example Remembrance Sunday and various Royal engagements.
Participation is voluntary - lots of Americans look at me strangely when I tell them I was a Cadet, I gather that in the US, a military school is for "problem children" as a last resort for desperate parents. Which is a shame; nothing beats an education by people who think 105mm artillery pieces are "light" and are happy for children to fire them, and are aware of the importance of being able to navigate by the stars, survive on the battlefield and fight armoured vehicles with other boys and girls. I haven't had to do any of that since I grew up, but I am glad I learnt to sew buttons and polish boots, and to take orders and to give orders.