In three tier architecture there will be 3 different layers 
like presentation tire, logic tire, data tire but in two 
tire the logic and data tire are combined.

Two tierarchitecture: The application and server present in
asame system this is called as two tie architechture

three tier architecture:The application and server present
in a differene systems  this is called as three
tierarchitechture


Plz correct me if iam wrong

2-tier architecture

In 2-tier, the application logic is either buried inside 
the User Interface on the client or within the database on 
the server (or both). With two tier client/server 
architectures (see Two Tier Software Architectures), the 
user system interface is usually located in the user's 
desktop environment and the database management services 
are usually in a server that is a more powerful machine 
that services many clients

3-tier architecture

In 3-tier, the application logic (or) process lives in the 
middle-tier, it is separated from the data and the user 
interface. 3-tier systems are more scalable, robust and 
flexible. In addition, they can integrate data from 
multiple sources. In the three tier architecture, a middle 
tier was added between the user system interface client 
environment and the database management server environment. 
There are a variety of ways of implementing this middle 
tier, such as transaction processing monitors, message 
servers, or application servers. The middle tier can 
perform queuing, application execution, and database 
staging. For example, if the middle tier provides queuing, 
the client can deliver its request to the middle layer and 
disengage because the middle tier will access the data and 
return the answer to the client
The most basic type of three tier architecture has a middle 
layer consisting of Transaction Processing (TP) monitor 
technology. The TP monitor technology is a type of message 
queuing, transaction scheduling, and prioritization service 
where the client connects to the TP monitor (middle tier) 
instead of the database server. The transaction is accepted 
by the monitor, which queues it and then takes 
responsibility for managing it to completion, thus freeing 
up the client.