Answer by Sharmilli Priyadarsini:
3-tier architecture and 2-tier architecture is not something restricted to .NET. These are architectural designs meant for sharing the load of the servers by hosting the tiers in multiple servers.
In case of 2-tier architecture, there will be an application tier and a data tier. The conventional window application that has the client application installed in your local computers and communicate with a centralised database located in some other machine is a good example for two tier architecture.
In case of 3-tier architecture, we usually have an application tier, business tier and a data tier. The application/presentation tier will have the UI related components like web pages and client interfaces. The Business tier will have all the business logics and the Data tier will have the database.
The communication between the tiers will happen in such a way that a reference to the tier will be added to another tier in order to create the objects of classes in other layer and access the members. In case of 3 tier architecture, presentation and data tier will not be able to bypass the intermediate business tier.