The Espada are known as the 'Swords' of the King. In those early days, shortly before the start of the Common Era when the first Vasto Lorde rose and took the title of King, ten powerful Arrancar rose alongside him as his faithful servants, champions of the guard and leaders of the forces of Las Noches. These first ten were the original Espada. In the time since, however, two more have been added to their number, and the composition has changed.
The strong wills required to become Espada frequently clash with one another, and after the apparent 'death' of the first King, it became clear that the Espada would soon tear Las Noches apart in civil war if not brought into order. The forgotten King returned in a new form, having torn his mask asunder, and forsaken his Hollow perfection in exchange for Shinigami powers. Having lost his Vasto Lorde status, the king resigned his post, and instead took up a new title, that of the 'Cero Espada' an Espada ranked above the others, who's primary purpose is to maintain order, and organize the Espada in times of need.
Unfortunately, not all Espada were ready to relinquish their plans so easily, nor were their followers prepared to join the coalition. As a result, the Cero was forced to appoint a twelfth and final Espada seat, the Gran-Exequia, or Grand Executioner. This Espada's purpose was to gather a fraccion (the Exequias), and track down any traitors to the crown among those in Heuco Mundo and to keep an eye on the other Espada's dealings. In essence, the Exequias serve the roles of Inquisitor, Federal Agent, and Executioner.
In the time since, things have changed. Between traitors killed, and the eternal war with the Shinigami and their pesky sidekicks the Quincy, the Espada have been unable to fill their ranks with Arrancar alone. As a result, any Menos or Arrancar of sufficient strength, and loyal to Las Noches can become one of the 'ten Espada.' The ranks of Cero Espada and Gran-Exequia, however, are reserved for Arrancar alone for now, though a future King may declare a change in that law. In order to take a seat among the Espada, a character must reach his second Power Stage, and contest against an existing Espada if the ranks are full.
Much like how each Shinigami captain has men under him, each of the twelve Espada have a 'fraccion,' a group of faithful followers whom they lead into battle, and who serve them directly. These Fraccion fill the role of Officers in the Gotei 13, providing power to the forces and leadership to the grunt troops. Even the King has a 'fraccion' of sorts, called the 'Royal Guard.'