A planet such as the Earth that has a melted iron core produces a magnetic field around itself, creating two magnetic poles.

By studying the iron minerals in ancient lava beds, scientists have determined that the location of the magnetic poles is not fixed, and changes around every 250,000 years, each time the two poles flip - north becomes south, and south becomes north. So if you say, hypothetically time travel some million years to the past, don't relay so much on your compass.

When the pole flip occurs, theory suggests that there's a temporary reduction in the entire strength of the magnetic field, causing the Earth's atmosphere to be less protected from the Sun's lethal radiation. This may cause climatic disruption, destruction of satellites, and eventually a loss of life, though not to the extent of extinction, since our ancestors have successfully survived these flips. Some claim that the magnetic field is only a small factor in the protection from lethal radiation, and the atmosphere plays the major part.

Currently there's a disagreement about how much time it takes for a pole flip to complete once it has started. Some argue it takes thousands of years, others say it's just a matter of weeks until the flip is completed.

Measuring the strength of the Earth's magnetic field in the last 100 years, scientists determined that there's a gradual decline. Given that a pole flip hasn't occurred in almost a million years, some independent researchers suggest that the decline is a result of a pole flip that has already started and is due to complete in the near future, which ranges from 'very soon' to '1000 years', a rather small amount of time in the geological time scale. Other researchers believe the decline of strength is just a passing fluctuation.

http://web.dmi.dk/fsweb/Projects/oersted/homepage.html (says andersa)

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