What is domain logic?

The domain is what you are modelling.

If you are modelling a business problem, they are the same thing.

If you are modelling something else, physics for instance, there is probably no business logic in your system, but the physics parts are still domain logic.

Domain is the world your application lives in. So if you are working on say a flight reservation system, the application domain would be flight reservations.

Business Logic on the other hand is a more discrete block of the entire Application Domain. Business Logic is usually a section of code built to perform one specific business process. So you would have business logic to take a reservation. Another bit of business logic would be code to refund cancelled tickets.

The objects that support your business process then become your business objects!