The client-server model uses a request-response pattern. The client makes a request; then, the server sends back a response. When you type a URL in the web browser, it sends a request to the server for the requested data. Then, the server responds with the appropriate data.
Before responding, the server may need to request data from another server. Databases and server-side files often store data. Let’s say a MySQL database has the data. The web server will send a request to the MySQL database, and MySQL will respond with the data. Once the webserver receives the database response, it can respond to the web browser with the retrieved data. This Web browser – Web Server – Database model is an example of a Three-Tier architecture.
Many web applications use three-tiered architecture. It is the most popular application architecture, so full-stack web developers must understand each tier.