It is fair to say that this is one of things that confuses people the most, I will try to simplify things here.
The category NPPL is a licence issued by the UK CAA (Civil Aviation Authority) to allow pilots to fly UK registered single-engine aircraft or microlights and self powered/launched motor-gliders within UK airspace. We are mainly concerned with the NPPL Microlight rating although it is possible to have ratings for all three category's if preferred.
So its one licence with three class ratings,
SSEA (Simple Single Engine Aircraft)
SLMG (Self Launch Motor Glider)
To obtain a NPPL with a Microlight Class Rating you must complete flight training with a UK Civil Aviation Authority authorised flying instructor entitled to instruct on microlights. The minimum age to start flying training towards the grant of the licence is 14 years. There is no maximum age. You can go solo at 16 and be granted your licence when you are 17.
The training syllabus can be downloaded from the NPPL web site or for a PDF copy click here. Details of the NPPL Medical requirements can also be found on the NPPL web site.
What is a microlight? Information taken from the BMAA (British Microlight Aircraft Association) web site
It's simple - a microlight is an aeroplane, capable of flight in the same way as any other. It is restricted to 2 seats, it must weigh around 265kg at most and it must be able to fly at low speed. Other than that, it's an aeroplane!
Actually the rules are slightly more complicated than that and a legal definition of a microlight can be found at the bottom of this page. There are three main types of microlight; the 3-axis (or fixed wing) type which looks more like a conventional aeroplane, the flexwing type which consists of a delta wing similar to a hang glider with a 'trike' unit suspended underneath it and powered parachutes which have a ram air canopy below which is suspended a wheeled power unit often similar to the flexwing type. All three of these types require a pilot's licence to fly in the UK.
A microlight aeroplane is one designed to carry not more than two persons which has a Maximum Total Weight Authorised (MTWA) not exceeding:
300 kg for a single seat landplane.
390 kg for an amateur built single seat landplane for which a UK Permit to Fly or Certificate of Airworthiness was in force prior to 1 January 2003
450 kg for a two seat landplane
330 kg for a single seat amphibian or floatplane
495 kg for a two seat amphibian or floatplane
315kg for a single seat landplane equipped with an airframe mounted total recovery parachute system
472.5kg for a two-seat landplane equipped with an airframe mounted total recovery parachute system
A microlight must also have a stalling speed at the maximum weight authorised not exceeding 35 knots calibrated speed. All UK registered aeroplanes falling within these parameters are Microlight aeroplanes.