Armies are a gameplay mechanic in Empire: Total War.
The army is the branch of a nation's armed forces charged with protecting a nation and its interests on land. Among military circles, an "army" can also refer to units larger than a corps, such as Napoleon's Grand Armee, or the US Army's 1st Army during World War II.
In both cases, armies are combined arms forces, typically including a variety of cavalry, infantry, and artillery forces along with their logistics and support elements (e.g. supply wagons, quartermasters). Armies in Empire: Total War often follow this structure, and, like real-world armies, are commanded by a General
Although their land-based nature imposes some obvious limitations, properly-employed armies can be extremely flexible and potent forces, capable for fulfilling a variety of missions.
The most clearest role of an army is that of direct combat, facing and defeating enemies in open combat on the battlefield. In turn, this tactical role can be used to reach a variety of greater strategic objectives, such as capturing territory, besieging cities or repelling invasions. Larger armies are generally better suited for this role, since they have more numbers and a variety of troops.
In addition to this role, armies can perform a variety of less confrontational missions. Garrisoned troops can act as a police force, reducing unrest in cities, while smaller, more mobile units can raid enemy outposts and land trade routes. Small, fast-moving units can also act as explorers, probing out-of-the way regions in search of an enemy's weaknesses.