[Oblarg]

Previous posts have already linked to in-depth explanations, so instead I'll just give a crude summary.

At any given moment, the "error" of your system is the difference between your set point and your current location.

The 'P' term contributes to output in a manner proportional to the current error. That is, if you are below the set point, the 'P' term will be positive.

The 'I' term contributes to output in a manner proportional to the sum of all past error. That is, if the sum of the error values at every previous iteration of your control loop is negative, then the 'I' term will be positive.

the 'D' term contributes to output in a manner inversely proportional to the rate of change of the mechanism's position. That is, if the mechanism is moving in the negative direction, then the 'D' term will be positive.

An easy way to think of this is that the 'P' term responds to where you are, the 'I' term responds to where you have been, and the 'D' term responds to where you're going.