Magnetism
A very common source of magnetic field found in nature is a dipole, with a "South pole" and a "North pole", terms dating back to the use of magnets as compasses, interacting with the Earth's magnetic field to indicate North and South on the globe. Since opposite ends of magnets are attracted, the north pole of a magnet is attracted to the south pole of another magnet. The Earth's North Magnetic Pole (currently in the Arctic Ocean, north of Canada) is physically a south pole, as it attracts the north pole of a compass.
A magnetic field contains energy, and physical systems move toward configurations with lower energy. When diamagnetic material is placed in a magnetic field, a "magnetic dipole" tends to align itself in opposed polarity to that field, thereby lowering the net field strength. When ferromagnetic material is placed within a magnetic field, the magnetic dipoles align to the applied field, thus expanding the domain walls of the magnetic domains.
Magnetic nozzle
If the strength of the applied magnetic field is sufficient, it magnetizes the light electrons in the plasma, which therefore describe a helicoidal motion about the magnetic lines. In practice, this is achieved with magnetic fields in the range of a few hundred Gauss. The guiding center of each electron is forced to travel along one magnetic tube. This magnetic confinement prevents the uncontrolled expansion of the electrons in the radial direction and guides them axially downstream. The heavier ions are typically unmagnetized or only partially magnetized, but are forced to expand with the electrons thanks to the electric field that is set up in the plasma to maintain quasineutrality. 
As a result of the ensuing electric field, the ions are accelerated downstream, while all electrons except the more energetic ones are confined upstream. In this way, the electric field helps convert the electron internal energy into directed ion kinetic energy.