Three-tier architecture in ASP.NET

• The presentation tier
• The business tier
• The data tier 

The presentation tier contains the UI (User Interface) elements of the site, and includes all the logic that manages the interaction between the visitor and the client’s business. (ASP.NET Web Forms, Web User Controls, ASP.NET Master Pages)
The business tier receives requests from the presentation tier and returns a result to the presentation tier depending on the business logic it contains. (C# Classes)
The data tier is responsible for storing the application’s data and sending it to the business tier when requested. (SQL Server Stored Procedures)