In 2 tier architecture we have only two tiers: client and 
server. Depends on the application consistency we called 
this 2 tier architecture as thin client-fat server 
architecture or fat client-thin server architecture. In the 
first one most of the business logic will be residing on 
the server side, where as in the second one most of the 
business logic will be residing on the client side.

In 3 tier or n tier architecture we will maintain one 
seperate tier for this business logic. we will call that as 
application server. This will act as the mediator for 
converting the client requests into the database 
understadable formats and it retrieves the data according 
to the command given by the client and it will again 
convert those results in the client understadable formats 
and displayes in the internet explores pages.

3 Tier:
We have 3 different layers
1. Presentation Layer
2. Application Layer
3. Database Layer

Example: SAP

2 Tier:
We have only 2 layers i.e 
1. Presentation Layer (Client System)
2. Application Layer + Database Layer (Server System))
Also called as Client Server Application