Three-tier architecture is a software architecture pattern that separates an application into three logical layers, each of which is responsible for specific functionality. The three layers are:

1. Presentation layer: This layer is responsible for presenting the user interface to the user and receiving input from the user. It typically consists of a web server, application server, or client-side application that communicates with the application's business logic layer.
2. Business logic layer: This layer contains the application's core functionality and rules. It processes requests from the presentation layer and returns data to the presentation layer for display to the user. It may also interact with data storage systems, such as databases or web services, to retrieve or store data.
3. Data storage layer: This layer is responsible for storing and retrieving data used by the application. It may include a database server or file storage system.

The three-tier architecture provides several benefits, including:

• Scalability: Each layer can be scaled independently of the others, allowing the application to handle increased user load as needed.
• Flexibility: Changes to one layer do not require changes to the other layers, allowing developers to modify and update the application more easily.
• Separation of concerns: Each layer is responsible for a specific set of tasks, which promotes a clean separation of concerns and improves maintainability.

Overall, the three-tier architecture is a popular choice for building scalable, flexible, and maintainable applications, and is widely used in web-based applications and services.