“Tier” can be defined as “one of two or more rows, levels, or ranks arranged one above another“.
1-Tier Architecture is the simplest, single tier on single user, and is the equivalent of running an application on a personal computer. All the required component to run the application are located within it. User interface, business logic, and data storage are all located on the same machine. They are the easiest to design, but the least scalable. Because they are not part of a network, they are useless for designing web applications.
2-Tier Architectures supply a basic network between a client and a server. For example, the basic web model is a 2-Tier Architecture. A web browser makes a request from a web server, which then processes the request and returns the desired response, in this case, web pages. This approach improves scalability and divides the user interface from the data layers. However, it does not divide application layers so they can be utilized separately. This makes them difficult to update and not specialized. The entire application must be updated because layers aren’t separated.
3-Tier Architecture is most commonly used to build web applications. In this model, the browser acts like a client, middleware or an application server contains the business logic, and database servers handle data functions. This approach separates business logic from display and data.So the 3 layers commonly known as:Presentation Layer(PL/UI),Business Logic Layer(BLL) & Data Access Layer(DAL).
Learn more about these architectures at Easy Lesson on 1-Tire vs 2-Tire vs 3-Tire – Winged Post