[EricH]

What you can do with a sensor depends on what the sensor is. Or, what sensor you use depends on what you want to do.

So, if you wanted to make your shooter automatically aim, here is what you would need, and what it would do:

1) Camera, to see the reflective vision targets. (You could also use an optical sensor, but those are a little more interesting to work with if you want to aim.) If you went to an auto-aim mode, the camera would be telling the robot which way to move the shooter. It could also be used to send a picture back to the drivers.
2) Depending on setup, I'd probably use potentiometers to check the distance each axis moved. Pots, as they are commonly known, count how far around a circle they have moved (normally...). Specifically, in this case, I would use the linear type of pot. An encoder would also work, but those tend to be used more for speed sensing than position sensing (a pot would have a hard time in the drivetrain, but encoders commonly are used there).

3) Other sensors commonly used in FRC:
--Gyro (angle--can be tricky to work with, IIRC)
--Accelerometer (linear acceleration)
--Microswitch (on/off switch, often used as an endstop)
--Ultrasonic (distance to an object)
--Optical sensor used as a beam break, often for "Is there something in my gamepiece handling mechanism?"