They're large, usually two KM in length. Their entire purpose is to cause the extinction of sufficiently advanced societies. The Citadel is basically a trap with a countdown, and a way for them to quickly jump to the seat of galactic government. When they attacked Earth in my dreams, London's population dropped 50% in a single week and it was a similar story world wide.
But the Citadel was dead. There's a lot of tech to salvage which the Agency can use, but Garrus and I were the only living things there.
Best case scenario was can jumpstart our species two hundred years. Worse case, we're in for a war. But we have a lot of powerful people.