Here we will see how to implement three tier architecture(also known as n-tier architecture) in asp.net application.Three tier architecture is a well known architecture in software development and you can implement it in web based as well desktop applications.
Three tier architecture basically means dividing your project into three layers
1) User Interface Layer [UI] (Also known as Application Layer or Presentation Layer)
2) Business Access Layer [BAL] (Also known as Business Logic Layer)
3) Data Access Layer [DAL]
The advantage of this functionality is it makes a logical separation of components so if in near future we want to replace / change our database we need to change only data layer or if we need to change user interface we need to change only user interface layer and rest remains the same.