2-tier architecture is Client server application and 3-tier architecture is Web based application.The user interface programs and application programs can run on the client side.When DBMS access is required, the program establishes a connection to the DBMS (which is on the server side); once the connection is created, the client program can communicate with the DBMS.in two tier architecture components are distributed over two systems: client and server.The advantages
of this architecture are its simplicity and seamless compatibility with existing systems.

an intermediate layer between the client and the database server in 3-tier architecture.This intermediate layer or middle tier is called the application server or the Web server, depending on the application. This server plays an intermediary role by running application programs and storing business rules that are used to access data from the database server. It can also improve database security by checking a client’s credentials before forwarding a request to the database server. Clients contain user interfaces and Web browsers. The intermediate server accepts requests from the client, processes the request and sends database queries and commands to the database server.