What is a Two-Tier Application?
A two-tier application is composed of a first tier — the user interface tier — that contains a user interface and business logic, and a second tier — the data tier — that contains all of the data access components. The user interacts with the interface tier, and uses it to perform all application-specific functions. The data tier contains all data sources and manages them so that they can provide all necessary data services to the user interface tier.