What is the effect on the air conditioning system if the windows or doors are opened, and what is the appropriate action if the room becomes to cold?

Air conditioning and heating systems are commonly used in both residential and commercial buildings, and are commonly controlled by thermostats. However, people using these systems may leave doors or windows open for fresh air or other reasons, therefore causing the air conditioning or heating system to operate at a nearly continuous level. This continuous operation of the air conditioning or heating system creates high electricity consumption and maintenance costs because leaving the door or window open will prevent the environment from reaching the temperature setting for the thermostat. This problem is especially acute in hotels and resort vacation rental facilities in which occupants are not responsible for, and probably are unaware of, high electricity, repair and maintenance costs. These costs are therefore borne by the owner or operator of the hotel, rental facility, or other building.

https://www.google.com/patents/US6196468

Does that mean the air conditioner would have to work harder in order to maintain the cooled environment?

If your a/c is set on auto, it will continue to bump and grind until it reaches the desired temperature. If the outside air is warming the room -- the a/c will remain on. If the ouside air is cooling the room -- the a/c will shut off once desired temp is reached. If the room becomes too cold -- Shut the system off or adjust the temperature.

https://en.wikipedia.org/wiki/Air_conditioner#Thermostats

https://en.wikipedia.org/wiki/Thermostat

And, yes...if it is warm outside and the windows/doors are open, you are putting an undue strain on your a/c system.

one more Q:
What are the appropriate action if the room becomes too cold? What are other methods to warm the room down?

THE AIRCONDITONER LOOSES ITS POWER TO COOL THE INSIDE OF THE CAR AS THE COLD AIR BLOWS OUT INSTEAD OF AROUND THE INSIDE OF THE CAR. SHOULD IT BECOME TO COLD JUST TURN DOWN THE ARICONTIONER

No, if you turn down a thermostat setting, you are going to make the air conditioner run longer and get colder in the room. If you want to warm a room that is too cold, you set the thermostat at a higher setting, i.e. 75 degrees vs 72 degrees.

Of course, there's always the simple -- but admittedly very low-tech and therefore possibly unattractive in this day and age -- option of just turning the air conditioner off and, if possible, opening the windows.

That is, in general, what most people I know do.

If the windows and doors are open, the Air Cooler WILL work harder and stay on continuously creating more cool air.

If the room gets to cold because the temperature is set too low, the A/C evaporator coil will freeze up and possibly fail causing a catastrophic break down of the unit in question.

Adjust the temperature dial/gauge/sensor/what-ever to a less cold setting when the room is unoccupied (such as off-business hours at the work place or away-from-home hours in the Domicile.

As roegar posted "down" means colder and "up" means less cool. We tend to say "turn the A/C DOWN" when it is too cool. "Down" meaning less power or effect.

It is only Human,

To add to what the original poster is saying, it is also recommended that proper insulation be installed. Having proper insulation is actually more important then the actual ac system. Another major factor is having a Copeland Compressor, the copeland compressor is considered to be the industry standard.

http://www.hvaclounge.com/index.php?threads/copeland-compressor.150/