Most industrial processes require that certain variables such as temperature, flow, level or pressure, remain at or near some reference value (setpoint). Closed-loop control is used to achieve this. The process controller looks at a signal representing the process value, compares it to the desired setpoint and acts on the process to minimize the difference (error). The method used by the controller to correct the error is the control mode. The four most popular control modes are on/off, proportional, integral and derivative.
Process Control Application Note (pdf) covers on/off control, PID control, tuning & signal conditioning.