A magnet creates a disturbance in the space surrounding it called the magnetic field. For a long time, magnets were the only known source of this magnetic field.

The existence of the field can be revealed by a force that acts on certain materials and on charges in motion.

A simple experiment to perform would be to sprinkle  iron filings in the neighborhood of the magnet. Each grain of metal becomes magnetized and, via mutual attraction, aligns itself along particular lines. These lines of force are also called field lines. They map out the magnetic distrurbance created by the sources present.

A loudspeaker is a system that transforms an electric signal into a mechanical signal. At the heart of this system we find a coil made of conducting wire, carrying an electrical current, which moves  under the effects of magnetic forces. A flexible membrane (or diaphragm), attached to the coil, acts like a piston that pushes the surrounding air. This animation has been slowed down many thousands of times -- audible signals lie between 20 and 20,000 Hz.

The magnetic field created by a magnet acts at a distance,   and can be observed using an array of compasses. The field is quite uniform between the two poles of the magnet.

The deviation experienced by a compass needle reveals the action at a distance of the magnetic field.    
Click on  the field  lines button to make the lines of force visible.

The Earth is a magnetized planet. The Earth's magnetic field is like that produced by a large bar magnet. The south magnetic pole coincides with the north geographic pole.

Note how a compass remains tangential to the field lines at every point.