What is the relationship between setpoint, process variable, and error in a control loop?

Setpoint (SP) is the desired value of the controlled variable set by the operator. Process Variable (PV) is the actual measured value of the controlled parameter. Error is the difference between SP and PV (Error = SP - PV for direct acting, PV - SP for reverse acting). The controller's job is to minimize error by adjusting the manipulated variable (controller output) that affects the process.