In a three-layer web application, there is physical separation between the presentation, application, and data layer, which is described as follows:

• Presentation layer: This is the server that receives the client connections and is the exit point through which the response is sent back to the client. It is the frontend of the application. The presentation layer is critical to the web application, as it is the interface between the user and the rest of the application. The data received at the presentation layer is passed to the components in the application layer for processing. The output received is formatted using HTML, and it is displayed on the web client of the user. Apache and nginx are open source software programs, and Microsoft IIS is commercial software that is deployed in the presentation layer.
• Application layer: The processor-intensive processing and the main application's logic is taken care of in the application layer. Once the presentation layer collects the required data from the client and passes it to the application layer, the components working at this layer can apply business logic to the data. The output is then returned to the presentation layer to be sent back to the client. If the client requests data, it is extracted from the data layer, processed into a useful form for the client, and passed to the presentation layer. Java, Python, PHP, and ASP.NET are programming languages that work at the application layer.
• Data access layer: The actual storage and the data repository works at the data access layer. When a client requires data or sends data for storage, it is passed down by the application layer to the data access layer for persistent storage. The components working at this layer are responsible for maintaining the data and keeping its integrity and availability. They are also responsible for managing concurrent connections from the application layer. MySQL and Microsoft SQL are two of the most commonly used technologies that work at this layer. Structured Query Language (SQL) relational databases are the most commonly used nowadays in web applications, although NoSQL databases, such as MongoDB, CouchDB, and other NoSQL databases, which store information in a form different than the traditional row-column table format of relational databases, are also widely used, especially in Big Data Analysis applications. SQL is a data definition and query language that many database products support as a standard for retrieving and updating data.

The following diagram shows how the presentation, application, and data access layers work together: