A brigade is typically a formation of three to five thousand soldiers. If it is a combat formation, it will usually mix infantry, artillery, combat engineer, and, if a "heavy" brigade, armor. An independent brigade, which is the largest unit in its area of operations, may be commanded by an officer of the rank of brigadier general. Some countries either call the officer of this level a brigadier, but not a "general". Other countries do not have a brigadier or brigadier general rank at all, but go directly to major general.
In Western militaries, where a brigade tends to be under a higher headquarters, it is usually commanded by a colonel. Brigades have a staff, with the sections designated with an "S" prefix, but does not have an officer designated as chief of staff, but rather as executive officer or deputy commander.
The combat unit of brigade size in the U.S. Marines is a regiment; a Marine Expeditionary Brigade has substantial air and support components, and is, overall, closer to the size of a small division.
In the current restructuring of the United States Army, the basic combined arms formation is to be the brigade combat team, with support brigades handling functions previously assigned to subcommands within a division. The next headquarters level above brigade is a flexible unit that is intended to control up to six combat and support brigades.
Historic use: brigade
The term "Brigade" is sometimes historic, as with the British Brigade of Guards. It may for traditional reasons, such as the student body at the United States Naval Academy being called the Brigade of Midshipmen, the only use of "brigade" in the United States Navy. It will appear in miscellaneous other contexts, such as the classical French restaurant organization being called a brigade.
"Regiment" is largely a traditional term; regiments were usually of a single arm (e.g., infantry or cavalry), rather than the more flexible structure of a modern brigade. U.S. Brigade Combat Teams may be formed around a traditional regiment, especially in the 82nd Airborne Division.
In the U.S. Army, combat arms battalions are commonly designated within the traditional regimental structure: 1/7 Cavalry was the battalion led to the Little Big Horn by George Armstrong Custer and later at the Battle of the Ia Drang by Hal Moore; 2/3 Marines were a reinforcement in the Battle of Khe Sanh, etc.
British tradition would sometimes make the regimental headquarters the "home", where new members were trained and then sent to battalions deployed in the field. A historic regiment might have only one active battalion.