AJAX in the wildAJAX is widely used around the internet; just look at the top 100 websites for some great examples of using AJAX. For example, Yahoo web search displays search term suggestions and topic meta data using AJAX, helping the user find information easier. The major web-based email clients (Gmail, Yahoo, Live Mail) are all powered by AJAX. If you are interacting with a web page, and it is fetching data without reloading the page, chances are you're using AJAX.
It's more likely that you'll use AJAX for discrete requests. You will often fetch each piece of data seperately - such as a list of recent items in one connection, and the data for each in many other connections. Because AJAX is asynchronous, you can have many AJAX requests pending at the same time, and handle each as they complete, instead of waiting for each request to complete before beginning the next.
Handling AJAX on the serverWhen you make AJAX requests, you are making fairly standard HTTP requests, either GET or POST requests. To serve AJAX requests on the server, you can simply output any standard HTML data.