Ferromagnetic materials are the most magnetically active substances in the world, and so they have very high magnetic susceptibilities, ranging from 1000 up to 100,000. These materials are made of atoms with permanent dipole moments, and when these materials form solids by exchanging electrons to make chemical bonds, something special happens. If the atoms are of the right type and if the bond lengths are right, the electrons discover that they can place the system in a state of lower energy by having neighboring atomic dipole moments aligned with each other. (This sentence probably makes no sense to you, but it is the best we can do. That's just the way quantum mechanics is.) If the entire sample were to be made of aligned dipoles, however, a strong magnetic field would be created, and this would be a state of high energy. So the system compromises. It makes microscopic regions in which billions of dipoles are aligned, satisfying the demands of most of the electron bonds. But the alignment directions of the separate regions are random throughout the sample, making a very weak net magnetic field. These regions are called magnetic domains, and their behavior gives ferromagnetic materials their distinctive properties.
For example, if a ferromagnetic material is heated to too high a temperature, it ceases to be ferromagnetic. The reason is that above a certain critical temperature, called the Curie temperature, the thermal motion of the atoms is so violent that the electrons in the bonds are no longer able to keep the dipole moments aligned. When this happens, the ferromagnetic material changes into a paramagnetic material with the usual weak magnetism.
The domains also make permanent magnets possible. If a ferromagnetic sample is placed in a strong magnetic field, the domains can be forced to coalesce into large domains aligned with the external field. When the external field is removed, the electrons in the bonds maintain the alignment and the magnetism remains. This means that ferromagnetic materials can remember their past magnetic history. This property of magnetic memory is called hysteresis and lies at the heart of audio tape, video tape, and magnetic disk storage for computers. The recording head of a tape recorder, or the write head of a disk drive, applies a field that magnetizes a small portion of the tape or disk. The magnetism in each portion remains until another magnetic field changes it. When each magnetized section is moved under the playback head of a tape player, or the read head of a disk drive, the moving magnetic field induces small currents which are amplified and turned into either music or data bits. If the domains were unable to remember the field that had been applied to them, none of this would be possible.
Diamagnetism
A diamagnetic material is one whose atoms have
no permanent dipole moment.
When they are placed in a strong magnetic field, Lenz's law acts on the
orbiting electrons and causes an atomic dipole moment to appear directed
oppositely to the direction of the magnetic field.
The effect is very weak, but its effect, roughly, is to cause repulsion where
other forms of magnetism give attraction.
Because this effect opposes the applied field, the susceptibilities of such materials are
negative, and because the effect is weak the magnitudes of the susceptibilites
are small, say in the range 
to 
.