In short, a Campaign is a way to organize and track your participants from an over-arching context beyond their interactions in a particular Application. Campaigns denote where your Applications and content are deployed. A Campaign can be a physical location, an event that you're exhibiting an Application at, or can even be a designated timeframe if you'd like to monitor an Application's use over a specific duration.
Each Campaign can contain multiple Applications, and each Application can belong to multiple Campaigns. This not only allows you to compare an Application's performance across different Campaigns, it also allows you to further distinguish between your Participants and gauge their needs and interests based on what Campaign they attended.