Short answers, pulled from the story.
Earth's magnetic field is generated by electric currents from convection currents of molten iron and nickel in the outer core, a process called a geodynamo. The convection is driven by heat escaping from the core, and the field originates in a region of iron alloys extending to about 3,400 kilometres deep.
The North Magnetic Pole, located in the Arctic, is really the south pole of Earth's magnetic field because that is where the field points downward into the Earth. A magnet's north pole is defined as the end that points north, and since opposite poles attract, it must be drawn to a south pole.
Earth's magnetic field deflects most of the solar wind and cosmic rays that would otherwise strip away the upper atmosphere, including the ozone layer that blocks harmful ultraviolet radiation. Calculations of carbon dioxide loss from Mars show that the dissipation of its magnetic field caused a near total loss of its atmosphere.
The most recent geomagnetic reversal, called the Brunhes-Matuyama reversal, occurred about 780,000 years ago. Reversals happen at irregular intervals, with gaps ranging from less than 0.1 million years to as much as 50 million years.
The Carrington Event, which occurred in 1859, was the largest documented geomagnetic storm. It induced currents strong enough to disrupt telegraph lines, and aurorae were reported as far south as Hawaii.
Earth's magnetic field has decayed about 10 percent over the last 150 years, and the dipole strength has fallen roughly 6.3 percent per century over the last two centuries. However, the present strength is about average for the last 7,000 years, and the current rate of change is not unusual.
Carl Friedrich Gauss first measured the field's strength in 1832 and developed the spherical harmonic analysis still used today. Modern models include the International Geomagnetic Reference Field and the World Magnetic Model, supported by satellites such as Magsat and Ørsted and a network of over 100 observatories recording since 1991.