At first, if you wanted data from the server to the browser in a session, you had to use an HTML Form. The HTML Form will send the request and then the reply will come to the browser as a new web page. Know that this approach has a lot of overheads and takes too long. Ajax is an approach to send the request in the smallest quantity possible and receive the response in the smallest quantity possible. By so doing, the speed is increased.
Use of Ajax
One of the use of Ajax as you have seen above is to increase the speed of transmission of data between the client browser and the server. If processing of the retrieved data is done at the client, your processing time at the server is saved; so the server is free to do other things. Another use of Ajax is that, in order to modify the web page with data from the server, you do not need a new web page.
At Client Only
Ajax technology takes place only at the client browser in a web page. There is no counterpart at the server. When the request is made at the client, it goes to a particular file (program) at the server. The program (file at server) looks for the result and sends it back as text to the same connection. So, Ajax sends the requests and waits for the result. Ajax finishes to do its task when it receives the result. The Ajax task is to make the request and receive the result.
At the server, any program (PHP, Perl, etc.) that can send the feedback (result) text is used.
By default, Ajax is asynchronous. This means that the Ajax task can be carried out, while other things are going on in the web page. For example, while the Ajax task is going on, other ECMAScripts can be doing animations. The processing of the web page does not have to stop, then Ajax carries its task and then the processing continuous. In that case, the Ajax task would be synchronous. You can make the Ajax task synchronous, by just changing the value of a parameter, but by default it is asynchronous. Being asynchronous is a good thing, because this allows other things in the web page to be going on, while Ajax takes its time to do its own thing. Ajax should not take too long though.
The tutorial series have been prepared in a step-by-step fashion. The code samples are well formatted and so very readable. There is no missing special character, as you would find in web pages of other sites. The links to the different parts of the tutorial series are easily accessible from each page. To start the tutorials, click: