A common advice on dealing with malicious users is:

Do not feed the troll!

What does this mean, what is the rationale behind this, and are there any exceptions?

Note: I am not asking this question because I am totally oblivious as to what the answer is, but because I was looking for a good basic and comprehensive reference on this important subject and failed to quickly find anything that did not already assume basic knowledge about this rule. I hope that this question can become this reference.