When you start your Army training, you'll be a Recruit. Once you've finished your Phase 1 training, you'll start your Army career as a Private. Your title may be different depending on which Corps you join - you might be a Trooper, Gunner, Sapper, Guardsman, Fusilier, Rifleman or even Kingsman.
A Staff Sergeant is sometimes known as a Colour Sergeant, and is a senior role where you'll rely on both your resource and man management skills. You may command a troop or platoon as well as being responsible for up to 120 soldiers.