A disc brake is the most powerful and effective friction based speed reduction system available. This type of brake employs a disc (aka rotor) and a caliper with contact shoes to apply stopping force to a wheel.
Rotor (aka the disc). The rotor is a thin steel or aluminum disc which connects directly to the hub of a wheel or to a drive shaft. When the wheel spins, the rotor spins along with it.
Shoes. The shoes are the soft metal pads which are designed to create predictable friction when they come into contact with the rotor. They generally are attached to the inside of the caliper.
Caliper. The caliper is a compression mechanism which "claps" brake shoes onto the rotor when the brakes are engaged. The caliper is generally attached to the frame or fork of the vehicle.
The characteristic which contributes the most to the performance of disc brakes is their consistency. Because the disc is separate from the wheel and tire, it will generally remain clean, smooth and cool. Since the rotor is a consistent interface, soft metal shoes can be used inside of the calipers to create an incredibly effective friction interface.
OEM parts. Many scooters come with an OEM disc brake setup. Although they are generally not as powerful as standard bicycle disc brakes, they generally perform well enough for a scooter or kart with stock equipment.
An exception to this general rule is the Goped Maddog caliper and disc system used on high-end Goped scooters. This system is very similar to common mechanical bicycle disc brakes and performs very well--even with other performance enhancing equipment.
High performance. Stock brakes tend to be only strong enough for stock speeds. Once a scooter is modified to reach faster speeds, more advanced mechanisms are needed. A common upgrade path is to use Industry Standard bicycle brakes. Due to the proprietary disc mounting patterns in use by many scooter manufacturers, adapters are often necessary to use IS discs. Also, many scooters use a standard caliper mounting pattern, but others may require fabrication in order to use bicycle calipers.
Disc brakes, once aligned correctly, generally require little or no maintenance for the life of a vehicle, but there are a couple instances where maintenance or repair could be necessary. The shoes inside the caliper which come into contact with the rotor while braking will slowly wear and need replacing. Most calipers use a standard shoe size, so replacements can generally be found at any bicycle shop.
In cases where extreme shoe wear has gone unfixed or a rock comes in forceful contact with the rotor, replacing the rotor may be necessary. If the rotor uses the IS attachment pattern, many rotor options are available. If the rotor users a proprietary bolt pattern, original equipment must be located.