Pickups and Your Sound:

Pickups are essentially magnets, and your strings are made of magnetic metals.  Your pickup creates a magnetic field that is disturbed when the strings move.  This disturbance is transferred to an electrical signal by your pickup, affected by all your guitar's electronics and eventually reaches your amp and is turned into vibrations which you hear as your guitar.  Pickups get their magnetism from either a magnet attached to their base, or from magnetic pole pieces.  Pole pieces are the metal cylinders that come out of the pickup under each string.  The pole pieces are wrapped in magnetic wire, which increases the strength of the magnetic field.

There are 3 main types of magnets used in passive pickups; Alnico II, Alnico V, and Ceramic.

Alnico II has the lowest output and has a smoother, warmer sound, with more bass than the other magnet types.  Alnico V is higher output than Alnico II and has more treble and midrange bite than Alnico II.  Ceramic is the highest output of all and the most treble attack.  In general, either Alnico II or Alnico V can sound good distorted or clean, but ceramic pickups generally produce a tone that isn't as pleasing clean, but somewhat preferred for heavy distortion.  Pro Guitar Shop highly recommends Alnico pickups for their "musical inherency."

Pickups for reference:

Alnico II: DiMarzio “Area ‘58" & “Area ‘61", Seymour Duncan “APTR-1 Tele Pickup”
Alnico V: Fender “Texas Specials” (SRV-style), Gibson “Burstbuckers”, Dimarzio “DP153 Fred Humbucker” (Joe Satriani-style)
Ceramic: Gibson “500T Humbucker”, Seymour Duncan “Duncan Distortion Humbucker”

Depending on the type of wire used to wind the pickup, it's thickness, how it was prepared and how old it is, the pickup's overall sound can be affected greatly.  In general, the more wire that is used will give you a greater output and a carry a heavier bass tone.

Single-coil pickups
The first electric guitars all used single-coil pickups.  A single-coil pickup has only one coil of wire.  It may have a single magnet, a single magnet with screws for adjustable pole pieces, or a separate magnet for each string.  Regardless of the number and arrangement of magnets, it is still a single coil pickup if it has only one coil of wire.

Unfortunately, in addition to producing an electrical signal from a vibrating magnetic field, a coil of wire is a very efficient antenna.  They will occasionally "pluck" electromagnetic radiation out of the air, most notably the sixty-cycle hum from building wiring, electrical noises from fluorescent lighting, and computer monitors.  Therefore, single-coil pickups are more susceptible to hum.

The single-coil sound
Single-coil pickups have a thin, clean, and transparent sound.  These pickups are usually about 3/4th of an inch wide and 2-1/2 inches long.  Single-coil pickups are common on Fender guitars such as the Stratocaster and Telecaster, two guitars that are very common in rock, country, and pop.

Humbucker pickups
A humbucker uses two coils and either two magnets, or pole pieces at opposite ends of a single magnet.  In a humbucking pickup, the two coils are wound with opposing electrical polarity, but the magnetic polarity for each coil is also reversed.  Essentially, this means that each coil carries two signals; the string vibration signal, which is reinforced, producing a thick sound, and the noise signal, which is cancelled.

The double-coil sound
The humbucker was featured on two models of Les Paul’s in 1957, the Goldtop Standard and the three-pickup Les Paul Custom.  The warm, smooth, double-coil sound of the Gibson Les Paul is a favorite for rock, blues, pop, and jazz.

Tips on Changing Pickups

Now that you know all you need about pickups and you've picked out the ones that are right for you, it's time to install your new pups.  First, you'll need to know that all your electrical components inside your guitar are connected by a metal called solder.  Solder is a mix of lead and tin, and melts at a relatively low temperature.  When melted over a wire and electrical terminal, it will harden in only a few seconds, and then you have a nearly permanent electrical link between the wire and the terminal.

If you want to change pickups, you'll need to get yourself a soldering iron, you can pick one up for as low as $8. The soldering iron should be in the 35 to 40 watt range.  If you get a low value soldering iron, it will take a longer to heat up solder to the melting point.  During that time the heat will travel through the whole component, and could warp the plastic and silicone parts inside.  If you have a hotter iron, it will be there for less time, avoiding this dangerous situation.  You’ll also need solder, industry standard is 60/40 rosin core solder, and will suffice.

To change your pickups, take off all of your strings, and unscrew the pickup you're removing.  Locate the wires from that pickup and carefully de-solder them by letting the existing solder melt, and then removing the wires.  Be sure when de-soldering or soldering anything that you let the component you're working on cool down before you work on it again, or you can overheat it.

Next, totally remove the pickup and screw the new pickup into the pick guard.  Follow the diagrams included with your new pickup.  Finally, test that everything works by plugging the guitar in, selecting the pickup you’ve added and tap on the pickup's coils with a screwdriver.  If you hear any noise, then you did it right, re-string and play.