Business objects are the base of workflows, they describe what a workflow is about and no workflow can be processed without a business object. A business object or business-relevant object is effectively a Purchase Order or an Invoice, actual documents on the SAP system which can be represented as a Business Object in workflow.
So what is a Business Object ? Does the Business Object representing a Purchase order contain any Purchase order data ? Is it a copy of an actual Purchase Order ? What would happen if the Business object Puchase order is changed, does the actual order also get changed ? Here's how it works: the Business Object represents a Purchase Order. It is not an actual order, it just represents one. So really all that is required is a link to the actual Purchase order, which would be the PO number. A business object has attributes (and key fields) which can hold any information on the Purchase order, so in effect all data could be held on the database (EKKO, EKPO, etc) as well as the Business Object - but - the developer would have to ensure the information on the Business object and real live object stays the same. The easiest way to avoid problems ? Don't copy information into the Business object. Fetch it from the original source - when required.
The Business object represents a real-life object. This means it's not the same. It is only a representation.