In the US military, a colonel in the Army, Air Force and Marine Corps is equivalent to a captain in the Navy; the insignia for all four positions is a silver eagle (hence the slang term "bird colonel" to distinguish a full colonel from a lieutenant colonnel). American colonels usually command infantry brigades and USMC regiments.
In the British forces, colonels are just below brigadiers. They are not usually field commanders, instead typically serving as staff officers in between field commands at battalion and brigade level. The insignia is 2 pips and a crown.