My defenition, IMHO, is that Cheese Tactics are any that take advantage of the program's flaws, poor AI, or programmer's logic.
The examples noted in the other responses all fit this.
The biggest example, again IMHO, is using the "fog of war" trick to fight big monsters. It is using a flaw in the program ("fog of war") and poor monster AI. (What dragon will stand there when a CloudKill spell starts?)
Now, I disagree with Throntar about the door. I can easily imagaine a party sending in an area spell, slamming the door, and blocking it with EVRYONE. Of course, if the door opens the other way, it doesn't work.
For those munchkins who must, cheese away.
But you'll never know the feeling of accomplishment gained from standing in the middle of the fray, wiping you brow and sword and seeing the fallen bodies of vanquished foes, and knowing that you did it.
I am reminded of the great Lombardi's thoughts:
"I firmly believe that any man's finest hour, the greatest fulfillment of all that he holds dear, is the moment when he has worked his heart out in a good cause and lies exhausted on the field of battle-victorious." - Coach Vince Lombardi
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The RudeDawg
Known in these Forgotten Realms as Perin LightEyes
and my girlfriends, Pamila and Pfil