Two tier architecture is a client-server architecture. In a two tier architecture the client talks directly to a server, with no intervening server(s). It has two hosts – the server and the client. Where the server controls both the application and the data. The client sends a request to perform a task to the server and the server responds by using the data stored on it and performs the task. It is mostly used where there will be small numbers of clients sending requests.
Three tier (also known as multi tier) architecture is a client, server and database architecture. It is more efficient than the two tier architecture because the server performs the task but the data is stored on the database. The client sends a request to the server to perform a task. The server then sends the request to the database. The database finds the relevant data and sends it back to the server. The server then processes the data and sends the result to the client. It is a longer process but the tasks are performed a lot more efficiently than a two tier architecture because, using the example in (iii) instead of the server processing all the students grades the server application is serving up a single grade (a single number, rather than many) which is a reduces network traffic in comparison to the two tier architecture. Three tier architecture is most commonly used for building web applications.
An example of a two tier architecture
A system that students in a school can use to find out what grade they are receiving in each of their classes. The program is structured so that a database of grades resides on the server, and the application resides on the computers in the school that...