AJAX stands for Asynchronous JavaScript and XML. It's a set of web development techniques that allows you to create interactive and dynamic web applications by making asynchronous requests to a web server from within your JavaScript code. The key concept here is "asynchronous," which means that these requests can be initiated and processed in the background without blocking or reloading the entire web page.

Before AJAX, when you wanted to update a part of a web page with new information, you typically had to submit a form or click a link, which would then result in a full page reload. AJAX revolutionized this process by enabling developers to send requests to the server in the background and receive responses in various formats, not just XML (as the name suggests), but also JSON, HTML, or even plain text.

Here's a basic outline of how AJAX works:

1. Event Triggering: An event (e.g., button click, timer, etc.) occurs in the web page.

2. AJAX Request: JavaScript code is executed that initiates an asynchronous HTTP request to a server. This request can be used to fetch data from the server or send data to the server for processing.

3. Server Processing: The server processes the request and generates a response. This can involve retrieving data from a database, performing calculations, or other tasks.

4. Response Handling: When the server's response is ready, the JavaScript code specifies how to handle the response. This often involves updating a specific part of the web page (e.g., a div element) with the new information.

5. Update the Page: The web page's content is dynamically updated without requiring a full page reload. Users can see new data or changes without interruptions.

Using AJAX, developers can create more responsive and user-friendly web applications. Popular applications of AJAX include auto-suggest search bars, real-time updates (like social media feeds), and dynamic content loading without needing to reload the whole page. While the term "XML" is part of the name, XML is just one of the possible formats that can be used to structure the data exchanged between the client (browser) and the server. JSON has become the most common format due to its simplicity and compatibility with JavaScript.