When customers are dissatisfied they will fall into one of three types of complaining customers:

Passive
Aggressive
Constructive

Let's take a look at each one.

Passive Complainers-
These are the worst kind as far as I'm concerned. They will complain to friends, family and anyone that will listen. They complain to everyone within listening range that is, except the one person who can do something about it, you.
Since you don't know there is a problem how are you supposed to do anything about it??
The only thing this type of customer does is keep you from making the situation better and providing the type of service they really want. (or at least claim they want)

Aggressive Complainers-
These complainers are difficult to please and are sometimes more concerned with displaying their emotions than in actually caring if you do something to address their problem or complaint.
They may shout, jump to conclusions, make unreasonable demands, and even make threats. Let them vent. Sometimes they just want to get it out.
Aggressive complainers should be handled by you staying calm and cool. If you can't do this, it's better to let someone else talk to them, or talk to them at a time once you've calmed down and aren't so mad yourself. (Even if it's only 15 or 2 minutes later)

Constructive Complainers-
People who are constructive complainers can actually help your business.
They will address problems or concerns in a calm, rational and often helpful manner.
Constructive complainers often allow you to see and understand what the problem is so you can make corrections that will be satisfactory to all involved.
For me I am most likely to go out of my way to help solve the problem for this type of complainer. To be honest I hate to even call them a complainer. I think of them more like a constructive criticism advisor. This type of customer can really help you make your business better.

No matter what type of complainer you have on your hands, do your best to deal with and handle complaints with the utmost care. Try to put yourself in the customer's shoes and see things from their angle if possible. Being an empathetic listener is key to help you help your customer achieve the results they are after.
If you deal with complaints in a timely manner and handle them effectively both parties should feel like they have achieved a win-win solution.