2 tier where client and server involved, 3 tier which is 
MVC framework

2-tier applications access database services or transaction
services directly from the client. here the application
logic resides on the client, making clients large and complex. 
3-tier client-server applications employs an intermediary or
middle tier, application server which operates between
client applications and the back end databases.
There are two fundamental motivations for using a three-tier
architecture over a two-tier
model:
* Improved scalability, availability, and performance
* Improved flexibility and extensibility of business systems

2-tier respresents client and server, in which server does 
the storage of data as well as the transaction. So the 
burden is more to the server ans the response may slow.

3-tier represents client, middle tier and server, where 
server does the storage, middle tier does the transaction. 
So burden for the server is less and the response may 
faster than 2-tier