in n tier aaplication a project is devided ito laters. 
commenly used 3 tier architecture used to devide layer into 
User interface, logical and business. User interface use to 
intract with end user. logical layer provide interface 
between Uers interface layer and business layer. logical 
layer is connected with database.
There is another 7 tier architecture. in this architure we 
use Configustion layer,common layer, data layer,Db layer, 
security layer, web and workflow layer.