Two tier architecture and Three tier architecture

Difference between Two tier architecture and Three tier architecture?

a. 1 tier application: All the processing is done on one machines and numbr of clients are attached to this machine (mainframe applications)
b. 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.
c. 3 tier application. Client are partially thick.Apart from that there are two more layers application layer and database layer.
d. 4 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.