Two-tier architecture is a client/server architecture , 
where a request to do some task is sent to the server and 
the server responds by performing the task.where as a three-
tier or a multi tier architecture  has client, server and 
database. Where the client request is sent to the server 
and the server in turn sends the request to the database. 
The database sends back the information/data required to 
the server which inturn sends it to the client.