Also known, in the Royal Marines and Infantry outfits when holding the appointment of CQMS, as a "Colour Sergeant/Serjeant", this is the most senior non-commissioned officer. Typical posts held would include company/battery/squadron etc Quartermaster Sergeant, troop/platoon sergeant (of a large or specialist troop or platoon), troop/platoon commander, spare dogsbody... the list is endless. Notably, the first technical supervisory appointment in The Royal Signals, The Royal Electrical and Mechanical Engineers and The Royal Engineers, to mention but three, tends to be at this rank - see Foreman/Yeoman of Signals, Artificer and Clerk of Works for examples.
The Staff Sergeant, depending upon cap badge and policy within that cap badge, may be either a high-speed, low-drag career meteor, destined for the top at dazzling speed, or a steady and reliable old plodder, awarded his crown as a reward for not being dead yet.
The rank badge is a rather fetching three-bar chevron, inverted, surmounted by a Queen's Crown.
Some Cavalry Regiments have Staff Sergeants but call them something else and give them promiscuous numbers of chevrons and other decorations. This is usually harmless and a matter of indifference to the rest of the Army.
NATO equivalent rank is OR-7, so inside the UK Armed Forces, our Staff Sergeant equates to a Chief Petty Officer or Flight Sergeant, externally to a Sergeant First Class (US Army), Gunnery Sergeant (USMC) or Hauptfeldwebel (Bundeswehr).