Electromagnetic fields are of various types, and each with its own specific characteristics.
The first large subdivision must be done considering constant magnetic fields and magnetic fields vary in time, or so-called buttons; the latter, to be divided into low frequency and high frequency.
The constant magnetic fields are those generated by permanent magnets or by electromagnets in which flows a constant DC current.
The permanent magnets are constituted by particular ferromagnetic alloys with high permeability; in practice, disks are -like small monete- that are applied to contact the skin for a certain period of time.
This kind of "magnetic therapy" is very studied and used in Japan, in the field of acupuncture is that of traditional medicine, where more research has been done in the 70's by Akio Yamada and Shuwichi Hirose of the Faculty of Medicine of the University of Tokyo .
This method is applied in the pains of skeletal muscle origin, both of inflammatory genesis traumatic, by placing the magnets directly on sore areas and those small skin areas that, when stimulated, produce distant pain.
The generated magnetic field is very high, from 3000 to 10,000 Gauss or more, and since decreases with distance from it in a way directly proportional to the square of the distance, the magnet should be much closer to the area to be treated.