Say you write a system which solves a business need for a customer.
The sum of all your code is the application logic, or system architecture - basically the entirety of the system you're building.
The business logic is the code subset which models and drives actual business processes. "What happens when an order for Product X is placed? How is the cost of Product Y calculated?" Ie. the bits of code where you probably need some input from the customer/domain expert/project stakeholder.
Ideally, the business logic is separated into its own tier or layer (see the Wikipedia article on N-tier architecture). The rest of the code can often simply be thought of as infrastructure to help that business logic execute (database wrapper, helper functions, service facades, external integration, GUI, etc).
