According to Perla, a war game is:
"A warfare model or simulation that does not involve the operations of actual forces, in which the flow of events affects and is affected by decisions made during the course of those events by players representing the opposite sides."The events-decisions reciprocal relationship appears to be the core of a war game. One could argue that this relationship is a big part of the appeal of war games.
Perla also explains that the dimensions of a war game are time, space, forces, effects, information, and command. Different off the shelf war games represent these dimensions differently. Time representation in war games can be turn based or continuous time based, space can take the form of hexagons, small squares or polygonal areas, forces can be represented as entities with attributes like amount of soldiers, movement speed, morale, ammunition (etc). In board war games, effects are the so-called "rules". Computer simulations take more or less the same approach, but many computer war games have "rules" way more rich and complex than
board war games. In some war games, transmission of information through the forces is restricted through a "fog of war" set of rules. Another set of rules applies to command (HQ units with limited command radius, disruption rules, etc).
One common opinion is that the more a war game's dimensions correspond with reality, the more valuable or useful the war game is.
"That's another thing we've learned from your Nation," said Mein Herr, "map-making. But we've carried it much further than you. What do you consider the largest map that would be really useful?"
"About six inches to the mile."
""Only six inches!"exclaimed Mein Herr. "We very soon got to six yards to the mile. Then we tried a hundred yards to the mile. And then came the grandest idea of all! We actually made a map of the country, on the scale of a mile to the mile!"
"Have you used it much?" I enquired.
"It has never been spread out, yet," said Mein Herr: "the farmers objected: they said it would cover the whole country, and shut out the sunlight! So we now use the country itself, as its own map, and I assure you it does nearly as well"
from Lewis Carroll - The Complete Illustrated Works.
As with every process we want to understand better via simulation, there is no substitute for reality. A simulation should focus the factors which are relevant to the process that is to be understood.