WHAT'S THE EARTH'S MAGNETIC FIELD LIKE?

The Earth has a magnetic field called the magnetosphere. It extends from the South Pole to the North Pole (it's as if the earth had a bar magnet in its centre). The North Pole is the South Pole of the Earth's magnetosphere.

Magnetosphere. By NASA (http://sec.gsfc.nasa.gov/popscise.jpg) [Public domain], via Wikimedia Commons

 
WHAT CAUSES THE MAGNETOSPHERE?
The centre of the Earth is made of liquid iron. This liquid moves slowly as the Earth rotates causing a magnetic field to form around the whole planet.

USING A COMPASS
A compass has a magnetised steel needle that is balanced inside a container. The north pole of the needle is painted red. It is attracted to the south pole of the magnetosphere: the magnetic north pole of the Earth. It will always point north.

Compass. By Evan-Amos (Own work) [Public domain], via Wikimedia Commons

WHY IS THE MAGNETOSPHERE IMPORTANT?
It deflects solar radiation from the Sun back into space. Radiation is harmful for plant and animal life on Earth.
It also helps us to navigate using a compass.