Mass Effect 3: Citadel is a DLC meant more as a way for the player to say goodbye to Commander Shepard's crew rather than expand on the events of Mass Effect 3.
The Normandy needs to go through some maintenance, so Shepard along with the rest of the crew are given shore leave. Soon things go awry, as an unknown faction recruits a group of mercenaries to kill Shepard. The main mission has a humorous tone, very different from the base game. Urdnot Wrex joins the fray and becomes a temporary squad mate. Once you have finished the main mission you can also set up battles in an arena with an expanded rooster of squad mates, which includes Jack, Miranda Lawson, Jacob Taylor, Grunt, Samara, Zaeed Massani and Kasumi Goto.
Shepard is also given a flat and, once the main mission is over, can organize a party. Squad mates can also be invited to the flat. They also propose you to participate in several activities around the Citadel.
The DLC also includes new enemies, three sets of armour, seven weapon modifiers and two weapons.