In order to understand this topic, you must first become familiar with how magnetic field is detected and measured.
A bar magnet,
when placed in a uniform magnetic field, will feel a torque
and attempt to "align" itself parallel to the field lines. The south pole
of the bar magnet will end up pointing toward the source of the field lines
(i.e. with the field lines pointing toward the south end). This detector bar magnet
is actually a compass needle.
The field lines B originate from the north pole of a larger external magnet. A uniform magnetic field can be formed between the north and south poles of a horseshoe magnet. When the compass needle aligns with the external field, the south pole of the needle ends up facing the north pole of the horseshoe magnet (opposites attract).
A horseshoe magnet or compass needle can be "made" from pieces of iron which have been magnetized. Iron atoms are actually miniature compass needles, with north and south magnetic poles that can align themselves with an external magnetic field. "Unmagnetized" iron is actually made up of randomized magnetic domains; each domain is a group of iron atoms with the same magnetic alignment, and adjacent domains are aligned in different random directions-- hence the overall material does not produce an external magnetic field. When the iron is brought near a magnetized piece of iron, it is attracted to it because some domains near the surface will always be able to align themselves with the external field. This explains how a cluster of metal paperclips can cling to the pole of a bar magnet as if all of the paperclips were magnets also. The alignment is temporary; when the iron is pulled away from the magnet, it will not itself attract another piece of iron.
To magnetize the iron object permanently, one must place the iron in a strong external magnetic field while also applying energy to the domains either by heating, or by strong vibration (striking the iron with a hammer, for example). The energy allows many of the magnetic domains to align themselves with the external field. Once the energy is removed, the aligned domains will remain aligned and the iron will then be permanently magnetized.
Iron or other magnetizable substances can be more easily magnetized when imbedded in something else. Magnetic media, such as audio and video tapes, floppy disks, and the stripe on the back of your credit card or student ID card, contain information in the form of patches of magnetization (aligned domains). The information is "layed down" (recorded) by running the medium through a changing magnetic field-- the field is changed in a controlled way such that a pattern of magnetization is produced that can be "read" (played back) later. If the medium is exposed to an uncontrolled magnetic field (such as near a microwave oven, a medical imaging device, or a metal detector), the information may be lost as the domains in the medium re-align. Erasure of magnetic media is accomplished by passing the medium through a steady magnetic field, so that all domains align the same way (no "information" pattern remains).
A bar magnet
produces field lines with a distinctive shape (the shape is "mapped" by
moving a tiny compass needle around and seeing which way it aligns). The Earth behaves
like a permanent bar magnet. The north geographic pole is actually the south magnetic pole.
When a compass needle is used for navigation purposes at the Earth's surface, the north
end of the needle points to geographic north because that location is the Earth's south
magnetic pole (attracts the needle's north end). The field lines converge at the poles
of the Earth, so they interact strongly there with charged particles in the atmosphere--
causing the so-called "Northern Lights".
All magnetism actually is the result of charged particles in motion (electrical currents). Even an iron atom is made up of charged particles in motion (electrons). See Magnetic Properties of Current.