What is the difference between open-loop and closed-loop control?

Open-loop (feedforward): control action is based on inputs or disturbances without measuring the controlled variable. Output is predetermined. Cannot correct for unknown disturbances or process changes. Closed-loop (feedback): control action is based on measured difference between actual output and desired output. Self-correcting - responds to any deviation regardless of cause. Most process control is closed-loop; feedforward is often combined with feedback for improved disturbance rejection.