A PID controller is a (closed) control loop feedback mechanism. It is used in industral control for proportional, integral and derivative feedback.
People at the Electronics stackexchange often ask about various ways to implement the PID controller itself, and ways to connect its output (a series of numbers), often using PWM and MOSFETs, to things a person may want to control -- heaters, motors, etc. There are discussions of windup and anti-windup, clipping, z-transforms, why scale-then-integrate is better than integrate-then-scale, etc.
People at the Robotics stackexchange typically assume you already have a PID implementation connected to some device, and ask about the subtleties of using it. Such as "loop tuning" -- ways of tuning and tweaking the Kp, Kd, Ki "constants", and the horrible things that go wrong when they aren't tuned right. How to deal with the fact that a PID for the elbow motor tuned perfectly when the shoulder is "down" isn't so perfect when the shoulder is "horizontal" or "up", or when the hand is empty vs. when the hand is holding a bowling ball, and ways to compensate. See http://robotics.stackexchange.com/questions/tagged/pid
