Campaign is central point of any online advertising. Campaign should always be focused on one particular product or service. More advance campaigns can be even focused on sub-product or sub-service.
For example you can have campaign for flowers shop. But also have another campaign only for tulips in mentioned flowers shop.
Campaign represents connection point between websites, zones and creatives. Campaign will display creatives in selected zones. Maybe it is not straightforward clear but one campaign can run on multiple zones and on multiple websites.
If we take as example flowers shop, then one campaign can advertise given shop in different zones of different websites.
In more advanced setups multiple campaigns can run in the same zone while advertising completely different services.