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: