Spartans are the ultimate super-soldiers of humanity. It is in no small part to them that humanity managed to survive the Human-Covenant War. It is their dark past, however, that makes them such interesting characters.
Every branch of the military, including ONI, have squads consisting of anywhere from a few to dozens of members. Spartans, while able to operate effectively individually, are normally grouped into fireteams consisting of between three to six members. These Spartan fireteams are the ultimate weapon for pinpoint force application in combat and non-combat scenarios.
The enlisted men and women of the UNSC get the job done, but they would never get a chance to fight were it not for the officers that command the ships and fleets that transport the might of the UNSC into combat. Likewise, officers would not wield any power were it not for the soldiers they command and the force those soldiers bring to bear upon the enemy.
The enemies of the UNSC are numerous, though each is unique in their objectives. Regardless of their desires, they all share one thing in common: A grave threat to the well-being of humanity and the lives of the UNSC women and men who put themselves between civilians and the enemy.
None were Spartans, some weren't soldiers, and some weren't even human, but all were instrumental in completing whatever objective they had.