Company – smallest unit of the army; generally consisting of 100* men led by a Captain.
Regiment – generally made up of 10 companies (1,000 men*) led by a Colonel; within the regiment, companies may be grouped into battalions.
Battalion – roughly half a regiment led by a Lieutenant Colonel.
Brigade – three to five regiments made up a brigade (3,000-5,000 men*) led by a Brigadier General.
Division – three to five brigades made up a division (9,000-25,000 men*) led by a Major General.
Corps – three to five Divisions made up a Corps (27,000 – 125,000 men*) led by a Major General in the Union Army and a Lieutenant General in the Confederate Army.
Army – made up of two to four Corps in the Union Army led by a Major General and made up of two Corps in the Confederate Army led by a General.
*Please note that these figures are at the optimum strength. Rarely, if ever, was a unit at full strength, especially on the Confederate side. A corps was very seldom more than 30,000 men.