The Fifth Column is a Star Wars: The Old Republic guild.
The GFC is a powerful underground movement that is active throughout the Galaxy. It has ties to interplanetary governments, militarizes, corporations, crime syndicates, resistance movements, and countless other organizations. Its mission is simple: galactic liberation from corruption through the removal or conversion of power-abusing individuals. The GFC is loyal to no flag, nor does it broadly target corrupt organizations as a whole except under special circumstances. The members of the GFC are incredibly diverse in their backgrounds, skills, and motivations. The majority of the GFC operatives come from the Republic, the Empire, or the galactic underworld.
Citizens of the Galactic Republic take pride in their democratic government and the ideals of freedom, peace, and justice that it stands for. However, bureaucracy and corporate influence impedes the democratic process. Corruption tarnishes the reputation of the valiant Republic Military. Even the Republic's legendary protectors, the Jedi Knights, seem to be powerless against these internal threats. Instead, many patriotic citizens, soldiers, and even Jedi have turned to the Galactic Fifth Column to bring the power-abusers to justice and restore the Republic's good name.
The Sith Empire stands for order and stability. Every citizen is a cog in the great Imperial machine: ensuring that through their combined and unwavering loyalty to the Empire, their Sith rulers will reward each according to his or her merit. The reality is that it is all too common for Sith and Imperial military officers alike to care more about advancing their own rank and status than about what is good for the Empire. Many of the Empire's true patriots, both Sith and Imperials, have turned to the Galactic Fifth Column to purge the Empire of these self-serving traitors so that the Empire may return to its former glory.
There are many sentient who wish to stand on their own two feet (or how many of whatever equivalent body-part they have) and make their own way in the galaxy without the overbearing laws or prying eyes of major galactic governments. But the underworld is also usually considered synonymous with gangs, violence, slavery, and every form of corruption. Many honest free-traders and honor-bound mercenaries have banded together along with the Galactic Fifth Column to combat the scum of the galaxy.
The GFC's structure and organization has changed vastly over time due to the need for the Column to adapt and evolve in order to remain competitive with its foes and on par with its allies. While many Columnists tend to operate independently most of the time, it is not uncommon for Fifth Column members to form groups with other members to pool resources and fill specialized roles. These groups can range from small cells with a handful of active members to large units with potentially hundreds of members. It is also not uncommon for Columnists to be active within more than one cell or unit. Almost all Columnists operate under self-assigned codenames to keep their true identities safe from their foes.