What is three tier architecture?

The three tier software architecture emerged in the 1990s to overcome the limitations of the two tier architecture.
There are three layers when we talk about three tier architecture:-
User Interface (Client) :- This is mostly the windows user interface or the Web interface but this has only the UI part.
Mid layer: - Middle tier provides process management where business logic and rules are executed and can accommodate hundreds of users (as compared to only 100 users with the two tier architecture) by providing functions such as queuing, application execution, and database staging.
Data Access Layer: - This is also called by the famous acronym "DAL" component. It has mainly the SQL statement which do the database operation part of the job.
The three tier architecture is used when an effective distributed client/server design is needed that provides (when compared to the two tier) increased performance, flexibility, maintainability, reusability, and scalability, while hiding the complexity of distributed processing from the user.