The Magnetic Field of the Earth

Like most planets, the Earth has a magnetic field surrounding it, generated by some not-entirely-understood process in the core. To first approximation, you can imagine the Earth as containing a giant bar magnet. That bar magnet is not aligned with the Earth's rotational axis, however; it is tilted. The northern terminus of this bar magnet (up in the Arctic) is called the north magnetic pole, and of course there is the south magnetic pole in the Antarctic regions of the Earth. A compass on the surface of the Earth will point towards the north magnetic pole.
The north magnetic pole is an
N pole S pole

A compass points toward the north magnetic pole; that means the N pole of the compass is attracted to the north magnetic pole. Unlike poles attract, and so the north magnetic pole is an S pole. Compare the Earth's magnetic field (with all the field lines pointing north, the way a compass would point) with the field of a bar magnet.

This is why physicists like to call them N and S poles instead of North and South poles: it's less confusing to say "the north magnetic pole is an S pole" than "the north magnetic pole is a south pole."

The Earth's magnetic field is a little more complicated than this:
• The north and south magnetic poles are not directly opposite one another: a line drawn from one to the other doesn't pass through the center of the Earth.
• The magnetic poles are moving, and rather quickly too. During the 20th century and probably before, the north magnetic pole was in northern Canada, but during this century it has moved into Siberian waters. The South Magnetic Pole is also moving northward. This is called polar drift, and it is due to fluctuations in the Earth's molten outer core.
• The Earth's magnetic field has often changed direction in the past, we can tell this by looking at the magnetization of ancient volcanic rock that cooled at various points in history. These geomagnetic reversals happen every 100,000 to 1 million years, though the last one occurred about 780,000 years ago. During a reversal, the magnetic field doesn't disappear, but may become chaotic, forming a number of poles.