The following may help (taken from [url="http://forum.ibanez.com/Default.aspx?g=posts&t=69722)"]http://forum.ibanez.com/Default.aspx?g=posts&t=69722)[/url]. Other factors affecting tone include the gauge of the wire (as well as the number of turns) and the winding pattern (which affects capacitance and harmonic content).
Manufacturers can determine the amount of output and tone a pickup has by altering two basic things:
1) The strength of the magnet. A stronger magnets creates a stronger magnetic field, and when that field is disrupted by the vibrating guitar string, a greater amount of flux is produced, generating a stronger signal. An unfortunate side effect of the stronger magnetic field is string pull. Extremely strong magnets can actually attract the strings and kill their vibrations, killing the sustain of the pickup.
2) The amount of turns of copper wire or the type of wire. Increasing the amount of copper or increasing its size will increase the resistance (impedance) and the inductance of the pickup, resulting in a higher output . A result of increasing impedance is the attenuation (loss) of highs in the signal. "Overwound" pickups are usually very bass and mid heavy and lack highs.
Pickup makers will experiment with these two factors (along with others) to change and create the sound of a pickup. For example, manufacturers will often use higher output ceramic magnets to retain some of the highs of their higher output pickups since the increased magnet strength will allow them to use less copper wiring, attenuating less of the highs.
One common misconception about pickups is that magnet have a direct effect on your tone. This is not true!!! The "Alnico magnets are warm, Ceramic magnets are harsh" theory is madness!! The role a magnet plays in the pickup is to create the magnetic field. The strength of the magnetic field determines the pickups OUTPUT, and different magnets can have different strengths. Tone is determined by the diameter and amount of copper wire, which creates the amount of inductance on the pickup. Remember, the higher the inductance and impedance, the less highs are put out. Magnets can have a small impact on some of the characteristics of the notes you play. For example, ceramic magnets have a higher output, which will generally create more punch and a sharper attack. Alnico magnets (depending on the type, there are 8) are usually weaker and have a softer, smoother attack, and may have a "spongy" feel. However, this has no effect on the amount of lows, mids, and highs sent out by the pickup, which is defined as tone. There are warm ceramic pickups and harsh alnico pickups out there.
Other factors affecting tone and output include construction of the pickup - eg. single coil (generally brighter and lower output) or humbucker (generally darker and higher output).