Guitar Electronics
An electric guitar may be wired in an infinite variety of ways. Generally, that standard controls offer a way to control to output (volume), tone and pickup selection. There are two common types of electronics for guitars; active and passive.
Passive Electronics
Passive electronics are what are supplied with most electric guitars. The term passive means that the controls themselves don't add anything to the signal put out by the pickups. This means that, when the volume or tone controls are set to their maximum level, they allow everything the pickups produce through to the amplifier. Turning the volume control down limits overall output. Tone limits the output only of certain frequencies (usually in the treble range).
Active Electronics
Active electronics include a preamp into their circuitry. A preamp is a low powered amplifier that boosts all or er that boosts all or parts of the signal before it reaches the amplifier. Amplifying the signal in this was boosts output and allows more control over the tone by giving guitarists the ability to boost specific frequencies (treble, bass etc.) in addition to cutting them. In order to boost the signal (or parts of it), some source of energy is required. Usually, a 9v battery is used to supply the extra juice.
Both types of electronics are good. Active proponents feel that these electronics give more control over their sound while many passive enthusiasts find active electronics sterile sounding. As with anything, it's a personal decision. Chances are most players will own both at one time or another.