In Multi-tier architecture/n-tier architecture the application is separated into n tiers-layers. The presentation tier or user interface, the business logic tier or business rules and the data tier are separate modules.
Using multi-tier architecture you don’t have to rewrite the entire application but just modify, upgrade, replace or add a tier-layer. In n-tier architecture each tier or layer is developed and maintained as independent module. Reusability, Manageability, Flexibility and Maintainability are just some of the benefits that this architecture provides. Using multi-tier architecture, large and complex projects can be divided into simpler projects. Divide and conquer!
In three-tier architecture/3-tier architecture, the most widely used n-tier architecture:
The presentation tier presents data to the user, displays information and results and permits data manipulation and data entry.
The business logic tier controls the application’s functionality, solves business problems and enforces business and data rules.
The data tier consists of DB servers. This layer interacts with persistent data usually stored in a database or in a file system.