In the Marine Corps there is more than a little difference between the Staff Non Commissioned Officer ranks of First Sergeant and Master Sergeant.
When a Gunnery Sergeant (E7) submits his promotion packages he must decide whether he wishes to become a First Sergeant or a Master Sergeant. These two ranks, while the same pay grade, represent two different career paths.
A Master Sergeant is considered an expert in his field. MSgts generally hold section chief billets and are responsible for overseeing training, operations, and maintenance. A Utilities MSgt who has spent his career as an electrician will have a wealth of experience in all manners of military power generation, wiring, generator repair, etc... MSgts become Master Gunnery Sergeants when they are promoted, and remain in their field.
The insignia for a Master Sergeant in the Marine Corps is 3 chevrons, 3 rockers, and crossed rifles in the center.
A First Sergeant holds a company level billet. The company 1Sgt is the company commander's senior enlisted advisor, the highest ranking enlisted man in the company. As DataJunkie states above the 1Sgt holds formations, instructs the platoon sergeants, etc... 1Sgt billets are not specialty specific. An infantry Marine that becomes a 1Sgt may find himself in an Engineer Support Battalion. Leadership is more important for a 1Sgt to possess than any specific technical skills. 1Sgts become Sergeants Major when promoted.
The insignia for a First Sergeant is 3 chevrons, 3 rockers, and a diamond in the center.
It should be noted that Master Sergeants can hold First Sergeant billets, but cannot become First Sergeants. This differs from the Army's rank structure which allows for a lateral move between the two ranks.