The 3-tier architecture is a design structure for web applications that separates the project into three distinct layers, each with a specific role. The first layer, called the client or presentation layer, is the user interface where users interact with the application, often through a web browser. The second layer, the logic layer or application layer, contains the business logic and processes incoming and outgoing data. Finally, the third layer, the data layer or storage layer, is responsible for managing databases and storing information. This separation into three layers allows for better organization, easier maintenance, and increased scalability of applications.