From BEAM Robotics Wiki
 Variable Resistors
Sometimes it is necessary to be able to easily adjust the resistance used in a circuit. In this case one of the many types of variable resistor would be used.
A potentiometer (slso called pot) a is a three-terminal resistor with an movable center connection. It can be thought of as an easily adjustable Voltage Divider.
 Trim Pot
A trim pot is typically a small, very simple device intended for mounting on PC boards. The main thing you'll want to be aware of when selecting a trim pot is cycle life. They are usually designed to last just a few cycles.
In the early stages of development, a BEAMbot will sometimes require more adjustments then a trim pot is intended to experience. Therefor it may be a good idea to avoid using trim pots, at least until the required value setting has been determined.
A rheostat is a two-terminal variable resistor, often designed to handle much higher voltage and current.
Any three-terminal potentiometer can be used like a rheostat by not connecting to the third terminal. It is common practice to connect the wiper terminal to the unused end of the resistance track to reduce the amount of resistance variation caused by dirt
 Resistive Sensors
Many sensors are also a type of variable resistor. Their resistance value will change depending on the level of the stimulous they are designed to detect.
For instance a Photo Sensors will exhibit an inverse relationship between its characterisc resistance and the level of light it detects. That is, the resistance across it will be lower when exposed to a brighter light source, and go higher as the light intensity is reduced.
Many Temperature Sensors exhibit a similar inverse relationship, becoming lower in resistance when the temperature goes up, and having a higher resistance as the temperature goes down.