1 tier application:All the processing is done on one machines and numbr of clients are attached to this machine (mainframe applications)
2 tier application: Clients and data base on different machines.Clinets are thich clinets i.e. processing is done at client side.Application layer is on Clients.
3 tier application.Client are partially thick.Apart from that there are two more layers application layer and database layer. ( Webservers follows 3 tier arch.., eg tomcat.... )
N tier application: Some clients may be totally non thick clients some cliints may be partally thick and further there are 3 layers web layer application layer and database layer. ( eg : Servers having business logic, which are application servers (eg weblogic, jbos......))
Please feel free to suggest me if any thing wrong...