What is the difference between Two Tier Architecture and Three Tier Architecture?

In Two Tier Architecture or Client/Server Architecture two layers like Client and Server is involved. The Client sends request to Server and the Server responds to the request by fetching the data from it. The problem with the Two Tier Architecture is the server cannot respond to multiple requests at the same time which causes data integrity issues.
The Client/Server Testing involves testing the Two Tier Architecture of user interface in the front end and database as backend with dependencies on Client, Hardware and Servers.
In Three Tier Architecture or Multi Tier Architecture three layers like Client, Server and Database are involved. In this the Client sends a request to Server, where the Server sends the request to Database for data, based on that request the Database sends back the data to Server and from Server the data is forwarded to Client.
The Web Application Testing involves testing the Three Tier Architecture including the User interface, Functionality, Performance, Compatibility, Security and Database testing.