From the Wikipedia article on wargaming:
Unit or map scale
Grand strategy — military strategy at the level of movement and use of an entire state or empire’s resources, with the focus being on a war (or series of wars) usually over a long period of time. Individual units, even armies, may not be represented; instead, attention is given to theaters of operation. All of the resources of the nations involved may be mobilized as part of a long-term struggle. This simulation almost always involves political, economic, and military conflict. At the most extreme end of this is the branch of strategy games in which the player assumes the role of an entire nation-state’s government where not conducting war is a possibility. Axis and Allies, Risk, and Empires in Arms are examples of this type of wargame.
Strategic — military units are typically division, corps, or army-sized, rated by raw strength. At this scale, economic production and diplomacy are significant. This sub-genre will often make use of all branches with the whole force of the engaging nations, covering entire wars or long campaigns.
Operational — common units are battalion to divisional size, carrying a value based on their overall strengths and weaknesses. Weather and logistics are significant factors, although a single army is the largest player-controlled element in most cases. Other branches of military force are more or less abstracted, having a mere campaign generally span the session of play.
Tactical — units range from individual vehicles and squads to platoons or companies, and are measured by the types and ranges of individual weaponry. A solitary force, others occasionally intervening, will usually cover the single battle or part of a large battle to be simulated.
Skirmish — represented by individual soldiers, units may have tracked wounding and ammunition count. A game is composed of a small firefight, introducing the “man-to-man” scale; the first of this type in the modern era of board wargames include Patrol and Sniper!. Early role-playing games were derived from skirmish wargames, and many are still played as such.