Ajax is a technique for building interactive web applications where certain parts of a web page are updated without reloading the entire page. The key benefits of Ajax include reducing response times, allowing asynchronous data retrieval and updates, and making web applications feel more like desktop applications. Some common uses of Ajax include autocomplete functions, live searches, and chat applications. While Ajax provides benefits like improved interfaces and responsiveness, it also has disadvantages such as pages being hard to bookmark and search engines not indexing dynamically updated content. Developers must also consider browser compatibility, network latency, and security issues when implementing Ajax.