Big badges. Also known as Sergeant-Major, or Corporal-Major in tewwibly tewwibly Cavalwy Wegiments. Divided into two classes, the senior being Warrant Officer Class 1 (WO1), and the junior being Warrant Officer Class 2 (WO2).
A Warrant Officer holds a Warrant, in the same was as a Commissioned Officer holds a Commission; except that a Commission is signed by the Monarch, while the Warrant is signed by the Secretary of State for Defence.
Try not to confuse the Rank (Warrant Officer) with the Appointment (Sergeant-Major). A subtle distinction......
Company Sergeant-Major, or CSM: Highest ranking WO in a Company.
Squadron Sergeant-Major, or SSM: Highest ranking WO, funnily enough, in a squadron.
Squadron Corporal-Major, or SCM: Same, but in a Cavalwy Squadwon. Not, god forbid, in the People's Cav.
Battery Sergeant-Major, or BSM: Go on, guess.
Academy Sergeant-Major, or AcSM: the RSM of RMAS. Scary.
Sergeant-Major Instructor: SMI: Runs courses.
Long ago, armies were formed into Companies of men; each Company had its Captain (with a meaning similar to the Captain of a ship), a Lieutenant (his deputy; a French word, meaning "place-holder"), and its Sergeant.
When the Regiments were formed into an Army, the Commander was a General; his deputy was a Lieutenant-General; and the senior soldier was the Sergeant-Major General.
More history apocrypha
Prior to WWII, during a shortage of junior officers, there was also Warrant Officer Class 3. Some of the Platoons in a Rifle Company would be commanded by a WO3, or Platoon Sergeant-Major (PSM). When this happens nowadays, the Platoon Commander is typically a WO2.