The Business Logic layer handles all of the business rules, calculations and actual logic within your application that makes it actually "do" things and it may often use some of the objects retrieved from your data-access layer. This layer would actually consist of more actual "code" and conditions that would be used to apply your "business rules".So you can basically think of data-access just as the name implies, it accesses and deals with sending and retrieving data from the database. The business logic layer on the other hand, deals with how you actually use some of the data from the database and ultimately what it can and cannot "do" within your application.

https://msdn.microsoft.com/en-us/library/aa581779.aspx

Business logic is a layer base technique which makes programming more secure and authentic. Business login use with four containers: UI(User interface) DO(Data objects) BAL(Bussiness access layer) DAL(Data access layer)

Generally,Business logic used in 3-tier architecture.in business logic, most of the business operation performs here. For example, after collecting form data we want to validate them with our custom business rule. Basically we define classes and business entities in this layer.

The term "business logic", in the context of web applications, refers to the functional requirements of a system which pertain to an organization's operating requirements. This compares with the "interface logic", used to define the mechanisms with which a person might interact with a web application, and "data model," used to specify the structure of and relationships among the different types of information pertaining to the application. An example of business logic would be a set of requirements where system users with a given permission are required to review and approve or reject submissions of a given type made by system users who have been given a different sort of permission. Another example would be a verification that credit card payments only be processed if a complete address is provided. In general, the proper development of business logic for web applications requires the understanding and approval of those individuals with managerial authority over a project. Otherwise, developers may implement a system based on assumptions which conflict with the actual business requirements of the organization.