Supposedly constructed by the long-extinct Protheans, this colossal deep-space station serves as the capital of the Citadel Council. Gravity is simulated through rotation, and is a comfortable 1.02 standard G's on the Wards and a light 0.3 standard G's on the Presidium Ring.
The Citadel was not, in fact, constructed by the Protheans; they were simply the last race to make use of the station before the asari arrived. It, and the mass relays, are the creations of the Reapers, and were built to help facilitate their cycle of galactic genocide. The Reapers were also responsible for the placement of the keepers on the station, in order that the occupying races would not discover the Citadel's key functions. The station is actually an inactive mass relay leading to dark space, designed as an elaborate trap so the Reapers can wipe out the heart of galactic civilization and leadership in a single, devastating strike.
At the height of the Reaper invasion in 2186, the races of the galaxy unite to construct the Crucible, a superweapon of ancient design with the power to destroy the Reapers once and for all. The final and most important component of the Crucible is the Catalyst, which is revealed to be part of the Citadel. Unfortunately, the Illusive Man learns of this and flees to the Citadel, where he alerts the Reapers of the galaxy's plans. The Reapers respond by seizing control of the Citadel and moving it into Earth's orbit to protect it. The station's arms are closed, sealing it off, thereby preventing the Crucible from docking. The Alliance leaders determine that the only way to get the Citadel's arms to open is to get someone inside the Citadel and manually open the arms.
During the battle for Earth, Alliance soldier Commander J. Shepard boards the Citadel via a transport beam and opens the Wards to allow the Crucible to attach to the station. Shortly afterward, Shepard meets the Catalyst - now revealed to be the Intelligence that controls the Reapers.
From this point onward, all communication with Shepard was cut off. It is still unknown what happened after Shepard met with the Catalyst. All that is known, is that the Crucible was never activated. The organic fleets outside the Citadel were unable to keep up with the reapers overwhelming presence, and were destroyed.
The Citadel is now thoroughly controlled by reapers, and it is being used as a sort of base of operations. To make matters worse, the reapers now have census records of more than half of the recorded population in the galaxy, and have classified starcharts to all known systems. This will undoubtedly make the harvest go by much more quickly.
While hundreds of capital ships swarm the exterior, reaper servitors have flooded the inside. There is no communication to the inside of the Citadel, but it is presumed that all 13 million of its inhabitants were killed by reaper ground forces.