A Proportional–Integral–Derivative (PID) controller is a control loop feedback mechanism used in application that requires continuously modulated control.

In a nutshell, a setpoint is where you want your process to be and an error is a difference between the setpoint and measure value. PID controller loop uses this error to correct the process so that the setpoint value can be met. For example, if you set your home thermostat to 23 degrees Celsius (setpoint), your furnace will turn on or off depending on the measure room temperature (error correction).

Purpose Of Having All Three… P, I and D
The screenshot below depicts how a control loop reacts if you only have one or the other.

Effects Of P and I
The screenshot below (from the Loop Simulator) demonstrates the effects of a P and I controller with too much or too little P and/or I set values. The process is typical with a dead time of 4 and lag time of 10. Optimal P and I is in red.
Note the response of the PID loops with P and I with very high or low values. Another thing to note is that you need to put your controller in manual and make changes with and output of 5% or 10%, and then put the controller back in auto.
P is in units of proportional band. I is in units of time/repeat. So increasing P or I, decreases their action in the picture.

Starting PID Values For Common Control Loops

These settings are rough, assume proper control loop design, ideal or series algorithm and do not apply to all controllers. Use Expertune’s PID Loop Optimizer to find the proper PID settings for your process and controller. (From Process Control Systems (Shinskey) p.99 and Tuning and Control Loop Performance (McMillan) p 39)

Loop Type PB
%
Integral
min/rep
Integral
rep/min
Derivative
min
Valve Type
Flow 50 to 500 0.005 to 0.05 20 to 200 none Linear or Modified Percentage
Liquid Pressure 50 to 500 0.005 to 0.05 20 to 200 none Linear or Modified Percentage
Gas Pressure 1 to 50 0.1 to 50 0.02 to 10 0.02 to 0.1 Linear
Liquid Level 1 to 50 1 to 100 0.1 to 1 0.01 to 0.05 Linear or Modified Percentage
Temperature 2 to 100 0.2 to 50 0.02 to 5 0.1 to 20 Equal Percentage
Chromatograph 100 to 2000 10 to 120 0.008 to 0.1 0.1 to 20 Linear

Reference