In two-tier architecture or Client / Server Architecture of one layers, as client & server is involved. The client sends the request to the server & the server responds to the request to fetch information from it. The problem with two-tier architecture is the server can not respond to multiple requests simultaneously that causes information integrity problems.
In the three-tier architecture or Multi Level three-tier architecture as a client, server & database are involved. In this, the client sends a request to the server where the server sends the request to the database for information on the basis of an application database server sends the information to server & information is sent customer.
Web application testing involves evaluating the architecture of level four, including the user interface, functionality, performance, compatibility, security & testing database.