The campaign is pretty much compressed into a single Epic-sized map. I know how that might sound, but the map is built to play just like any normal campaign.
For example, you'd play a level, see a clip, then open to a completelly new stage in a campaign. In this, you'd play a level on an area on the map, then upon completion see a clip, then fade to a title and completelly new level elsewhere on the map.
So basically it feels like and plays just like a campaign, but doesn't use multiple .w3x files. It's decently effective, and as far as I know the only way to do this online.