Okay, in a nutshell a wargame is a game about war. For a more detailed discussion on what is wargame, try J Dunnigan's A Complete Wargames Handbook . If you don't know, he's the granddaddy of commercial wargames. Generally, the wargame consists of a mapboard and cardboard counters. The mapboard represents the area overwhich the conflict is fought.
Counters represent the forces that took part in the conflict.
Cardboard counters are the basic playing pieces of a wargame. They represent military formations of varying size, depending on the game in question. Printed on the counters are typically the name and type of the unit, combat strength, movement allowance, etc. Other counters serve as markers to denote fortifications, supply, special circumstances like demoralisation, isolation, disruption and so on. The latter kinds are usually termed as markers.