what are the types of sensors, and how does each serve a purpose?
Take a look at this page here for the VEX specific sensors, Click.
Under each sensor is a short description and there is more info in the Inventor’s Guide PDF on each sensor page, click on the sensor to get to that page.
If you have any more specific questions post them.
Have you seen the page, Vex Sensor Products? There is a good description for each type of Sensor.
Also if you want to try something, and are not sure what sensor to use, just ask here. A good example is the Thread, Programming the Vex Potentiometer. A little discussion about the Problem and Potential Solutions, resulted in a change in the Implementation of the chosen Sensors.