The most popular accessory decoder are for turnout or track switches. There are two types.
1) COIL BASED
One or two coils are use to drive the switch points to one of two position. Coil based machines typically have a "Snap" sound when the coil is energized due to the very quick action of moving the point rails. The most popular machine is called a "Twin Coil".
2) MOTOR BASED
A motor is used to move the point slowly from one position to another. The speed and sound of the machine will vary with the design.
Click on the page below for the type of turnout or switch motor you have for more specific information: