Earth's Magnetic Field :

Earth's geo-magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind ( A flow or stream of highly-accelerated particles emanating from the Sun).
The range of Earth's magnetic field is about 25 to 65 micro-tesla(µT) which is about 0.25 to 0.65 Gauss.
The Earth's magnetic field is generated by the motion of molten iron alloys in the Earth's outer core (The geodynamo).
The North Magnetic Pole wanders off, but it does too slowly that we can use a simple compass for navigation.

Here comes the interesting part.
Over several thousand years, the Earth's geo-magnetic field reverses.Current North Pole becomes the south pole and our current south pole becomes the North pole.
Yep.
Geo-Magnetic Reversals occur once every several thousand years.


Why is the Magnetic Field so important? 
The magnetic field protects Earth from highly-accelerated charged particles like the electrons or the ions from the Sun.
Without the magnetic field, Earth would have been riddled by these particles and would the core of the earth would de-stabilise. The region over the ionosphere is called the magnetosphere, and extends several tens of thousands of kilometers into space. This region prevents the cosmic rays from entering into Earth.
Without the Magnetic field, the entire ionosphere would be stripped off taking away the Ozone layer too which would cause harmful UV rays to enter.

Declination from 1590-1990.