Militia are a type of militia in Empire: Total War.
These musket-armed troops are recruited to defend their own locality, not carry the war to an enemy's land.
Militia or provincial troops are commonly held to be inferior to regular soldiers and, although they are trained to use the same tactics as marching regiments of the line, there is some truth in this assertion. Militia are expected to act as reassuring presence, and sometimes as a police force in suppressing local disturbances.
Historically, it was not unusual for militia to be locals recruited as part-time soldiers while retaining their day jobs and trades. In Britain, for example, service in the militia was seen as a good idea: you not only looked very patriotic for volunteering, but you could not be sent overseas! For ambitious would-be officers, there was a hidden financial benefit to joining the militia. A commander who could persuade his men to transfer to the army with him would not have to pay the cost of his regular commission.
Militia are cheap infantry available to most Western nations, with one exception: Russia fields Cossack Infantry and Gorodskiye Streltsy in their place. The Eastern nations have assorted units that all qualify as militia equivalents.
Militia, and their equivalents, are useful in the early game as cheaper alternatives to line infantry (despite their inferior statistics). They become less effective, however, when more nations field regular line infantry in their armies: militia are inferior to line infantry in every way, and this difference only becomes worse when more military technologies are researched. Militia do not benefit from the Fire by Rank technology, effectively meaning they have only a third of the firepower (not including their inferior accuracy) of line infantry when this technology is researched; in addition, they cannot utilize the Square Formation, making them vulnerable to cavalry.
Like some other units such as artillery and light infantry, militia gain the option to pre-deploy some defensive measures on a battlefield should they remain in the place on the campaign map for more than two turns: defensive barriers behind which militia can shoot and crouch. This grants militia much more resistance against enemy musket fire, and somewhat slows and weakens charges. Militia also have access to anti-cavalry wooden barricades.
Their lack of battlefield prowess addressed, militia are not wholly without utility. They make useful garrison troops, cheaply and efficiently suppressing public unrest. They can also make a useful emergency force, holding regions which have unexpectedly come under attack until regular troops can arrive to relieve them.
In multiplayer, some generals elect to use militia as a "meatshield," using the cheap militiamen to absorb enemy fire which would otherwise hit the more valuable troops marching behind them. Although somewhat cold and calculating, a well-implemented meatshield can be remarkably effective, ultimately saving lives and winning battles.