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.

In other word you can say that in 3-tier architechure 
business logic,databases & presentation are maintained in 
different files whereas in 2-tier architechure business 
logic &databases are maintained in same layer or file
3-tier architechure in other word u can say fallows MVC 
architure