I have due regard to the answers provided by CP Seniors.
There is a lot to the tiered architecture for which lot of pointers were already provided in the above answers.
I want to give a general description of 3 Tier Architecture.
Broadly speaking 3 Tier Architecture is to divide the application into 3 tiers or layers as Data layer, Business layer and Presentation or GUI layer.
The purpose is to maintain a loosely coupled system. For example the presentation layer is say developed with Windows Forms and later you want to implement ASP.NET Web Forms, you can easily create one more project with ASP.NET in presentation layer and the Business and Data Layers can be used with the newly created ASP.NET web forms. Similary, say initially MS Access is used for Data access and later you want to switch to SQL Server, it can be achieved with out disturbing the Business and Presentation or GUI layers.
I think the following article may be useful
3-tier architecture in C#[
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