Magnetic fields are produced by magnetism, the flow of electric charge, or fluctuating electric fields. In the first case, the magnetic field can be described as a region around a magnetic object, such as a magnet:

Inside the magnet, the magnetic field B is directed from the south (S) to the north pole (N). Outside the magnet, the direction of the field is represented by imaginary force lines called magnetic flux lines, which always flow in a closed loop from the north pole to the south pole. The density of these lines define the strength of the magnetic field, i.e., the more lines, the stronger the magnetic field. Magnetic field sources are always dipolar in nature, having north and south magnetic poles. But unlike electric monopoles--or charges--magnetic monopoles do not exist, i.e., an electric dipole consists of positive and negative...