Permanent and induced magnetism, magnetic forces and fields

Every magnet has two poles, north and south, where the magnetic forces are strongest. Like poles repel each other (north–north or south–south); unlike poles attract each other (north–south). The force between two magnetic poles is a non-contact force — it acts even when the magnets aren't touching.

The force between a permanent magnet and an induced magnet (e.g. a piece of iron) is always one of attraction, never repulsion — this is how you can tell a permanent magnet apart from an induced one experimentally: bring two suspected magnets close together from both orientations; if they only ever attract (never repel), at least one of them is an induced magnet, not a permanent one.

Magnetic field lines are drawn from the north pole to the south pole of a magnet, on the outside of it. At any point, the direction of the field line shows the direction of the force that would act on a north pole placed there. The field is strongest close to the poles, and gets weaker further from the magnet — shown on a field diagram by field lines that are closer together (stronger) or further apart (weaker).

A plotting compass is a small compass that can be placed at different points around a magnet; the needle lines up with the field at that point, so moving the compass around and tracing its direction at each point builds up a picture of the field's shape.

The Earth itself behaves as if it has a giant bar magnet at its core, producing a magnetic field that extends into space — this is why a freely suspended compass needle lines up with the Earth's field and points towards geographic north (in reality, close to, but not exactly at, the Earth's geographic North Pole).