An arcade gas pedal (the kind used in driving video games), may use one of several different types of interfaces to send its signal to the game. Most of these are ananlog in nature, although digital ones are used in some of the newer titles.
These controls are rather expensive when compared to standard joysticks and pushbuttons, and people rarely implement them into multi-game projects (such as quick switch JAMMA machines, or MAME cabinets). But they are the only way to play a driving game, while still maintaining the proper feel.
There seem to be two types of these. The first type spins an encoder wheel in much the same way a mouse or trackball does. While the second type works by sending a beam between two poles, which is broken by a third pole attached to the pedal. The game Turbo uses this second type, and other games probably use it as well.
An analog arcade pedal works in much the same manner as any analog joystick. Pressing down on the pedal rotates a POT inside the machine, which is read by the game. These are fairly common in older games, and can be easily repaired with off the shelf components, as their design is very simple.
Three position digital pedal
This is a very cheap design. The basic idea is that inside the game are two microswitches. One at each extreme of the pedals movement. This provides for three different levels of acceleration (or lack of in the case of a brake pedal). When the pedal isn't pressed down at all, the top microswitch will be in the closed position, when it is fully depressed the bottom switch will be in the closed postion. Anything else leaves both switches open (which gives the pedal 3 possible states). This design is fairly popular, despite its inherent weaknesses, and lack of subtlety. It is used in Special Criminal Investigation and a variety of other driving titles.
Full digital pedal
A full digital pedal will have a large number of distinct positions to simulate analog operation. This offers all the advantages of an analog or optical pedal (well almost, 32 or 64 positions is nearly as good as true analog operation, at least for a video game it is), without the high failure rate of those devices. These pedals are fairly expensive, and are common on newer dedicated driving games.