The earth's magnetic field is generated by the motion of molten iron in the earth's core. The combination of convection and coriolis forces generates the right set of currents to establish a dipole field with the north and south poles approximately in line with the rotational north and south poles. We partially owe our existence to this magnetic field because it provides a shield against charged particles from the solar wind. Without it, we would be subject to ionizing radiation and may also lose our atmosphere.
Neither Mars nor Venus has a significant magnetic field and we're not fully sure why. Mars once had a thick atmosphere of CO2 that may have been blown away by the solar wind. It could be that the molten iron has solidified or that the pattern of flow no longer supports a magnetic field. In any case, I think the lack of a field on Mars should make us less secure that we'll always have ours.
The earth's dipole flips every 250,000 years on average. We're not exactly sure why but some recent magnetohydrodynamic simulations of the geodynamo have shown examples of field reversals due to instabilities in the turbulent flow. It's been 780,000 years since the last reversal so we may be due for another one soon. It will take approximately 4,000 to 10,000 years for a reversal to take place. During the transition, the magnetic field may lose its intensity as well as its dipole structure which may have implications for life on the earth.
Another consequence of the turbulent geodynamic flow is that the north pole is in constant motion. It usually moves about ten kilometres a year but recently it has been moving forty! If it keeps moving at this rate, in about fifty years it will leave Canada and reach Siberia. This increased speed of drift of the north pole may be nothing more than natural random variations but it definitely makes me worry just a little.