The camp itself consists of several tents. Multiple catapults, controlled by one Stormcloak Soldier each, are launching flaming projectiles towards the city.
Regardless of which side the Dragonborn chose during the Civil War, the camp will be there. However, there will only be a marker for it if the Dragonborn sided with the Stormcloaks. After the battle the camp will be deserted, save a single Stormcloak Soldier residing in the command tent.