Campaigns are a series of missions of different scenarios which usually have a minor narrative connecting them. A campaign often has a complex final mission. There are mandatory missions and optional missions. Optional missions are not required to complete the campaign but count toward the total number of stars earned in a campaign. There are a number of differences to gameplay from standard single missions such as perma-death, ironman mode also has additional gameplay changes.
Differences exist between doing single missions and playing campaign missions. The most noticeable is the death and injury of officers. If a trooper dies on a mission and the player decides to continue, that trooper is KIA. KIA troopers cannot be replaced and therefore cannot be used until they're replaced with a new trooper after a campaign. If an injury occurs a trooper will be injured at the same seriousness for the next mission. An injured trooper will heal to full health whether deployed or not, unless further injury is sustained.
Ironman mode generally makes the gameplay more risky and makes each mission's stakes higher. A mission cannot be restarted, therefore the player's first attempt is their only attempt. The benefit to Ironman mode is the player receives double XP and higher star reward when finishing a campaign.