Ajax (Asynchronous JavaScript and XML) development refers to a web development technique used to create interactive and dynamic web applications. By using Ajax, a web application can send and retrieve data from a server asynchronously (in the background) without interfering with the display and behavior of the existing page. This results in a more fluid, responsive, and user-friendly web application experience.

Key aspects of Ajax development include:

1. Asynchronous Requests: Ajax allows the browser to communicate with the server without reloading the entire page. This is typically done using the XMLHttpRequest object in JavaScript.
2. JavaScript and the DOM: JavaScript is used extensively in Ajax to dynamically display and interact with the information presented to the user. The Document Object Model (DOM) is manipulated to update the web page content based on the server’s response.
3. JSON and XML: Although XML was traditionally used as the data format in Ajax, JSON (JavaScript Object Notation) has become more popular due to its lightweight nature and ease of use with JavaScript.
4. Server-Side Scripting: Server-side scripts handle Ajax requests and send responses back to the client. These can be written in various programming languages like PHP, ASP.NET, Java, or Python.
5. HTML and CSS: HTML is used for structuring the content of web pages, while CSS is used for styling. Ajax interacts with both to dynamically update the UI without a page reload.
6. Handling Responses: Ajax development involves handling responses from the server and updating the web page accordingly. This could include displaying new data, updating existing elements, or even navigating to a new page.
7. Fallbacks for Older Browsers: While modern browsers support Ajax natively, developers often include fallbacks for older browsers or those with JavaScript disabled.
8. Security Considerations: Security is a crucial aspect of Ajax development, as it involves client-server communication. Common practices include validating input data, using secure HTTPS connections, and protecting against cross-site scripting (XSS) and request forgery.
9. Frameworks and Libraries: Many JavaScript frameworks and libraries, like jQuery, AngularJS, React, and Vue.js, provide simplified methods for implementing Ajax.
10. User Experience and Performance: Ajax is used to enhance the user experience by reducing wait times and improving the interface responsiveness. Developers must also be mindful of potential performance issues, such as the overhead of frequent server requests.

Ajax has been a significant driver in the evolution of web applications, transitioning them from static pages to dynamic, interactive experiences akin to desktop applications. It’s widely used in various applications like real-time form validation, search suggestions, interactive maps, social media platforms, and many other web applications where real-time data update is crucial.