A brigade is a military unit that is typically composed of two to five regiments or battalions, depending on the era and nationality of a given army. Usually, a brigade is a sub-component of a division, a larger unit consisting of two or more brigades; however, some brigades are classified as a separate brigade and operate independently from the traditional division structure.
Traditionally, a brigade's commanding officer was a brigadier general. In most modern armies, a brigade is now commanded by a colonel.

Origin
In the British Army, the brigade has been the smallest tactical formation for more than two centuries, since regiments are either administrative groupings of battalions (in the infantry) or battalion-sized units (in the cavalry). A typical brigade may consist of approximately 5,500 personnel between two mechanized infantry battalions, an armored battalion, an armored artillery battalion, and other logistic and engineering units.
The Canadian Forces currently has 3 Regular Force Brigade Groups, known as Canadian Mechanized Brigade Groups: 1 CMBG, 2 CMBG, and 5e GBMC, the primarily French Canadian Brigade Group. These CMBGs are each composed of two mechanized infantry battalions, one light infantry battalion, one armoured regiment, one mechanized artillery regiment, one engineer regiment, one combat service and support (CSS) battalion, and one Military Police Platoon. Regular Force CMBG strengths are 4,000 personnel. Canada also has 10 Primary Reserve Brigades (Canadian Brigade Group), 31 CBG through 39 CBG, and 41 CBG. The CBG formations are for administrative purposes and, as such, are not deployable.
In the United States Army, a brigade is smaller than a division and roughly equal to or a little larger than a regiment. Strength typically ranges from 1,500 to 3,500 personnel. Army brigades formerly contained two or more and typically five regiments, during the American Civil War, but this structure is now considered obsolete.
In the United States Marine Corps, brigades are only formed for certain missions. Unlike the United States Army, the Marines have intact regimental structures. A Marine brigade is formed only for special expeditionary duty, for which it is outfitted like a smaller Marine Expeditionary Force (MEF). For example, TF TARAWA (2d MEB) during the Operation Iraqi Freedom campaign.
In the armies of colonial powers, such as the British Empire, brigades frequently garrisoned isolated colonial posts, and their commanders had substantial discretion and local authority.
The typical NATO standard brigade consists of approximately 4,000 to 5,000 troops.