Synchronous requst in Ajax

To start with I would say what is synchronous: It is when the script stops and waits for the server to send back a reply before continuing with your code and on the other hand asynchronous: where the script allows the page to continue to be processed and will handle the reply if and when it arrives.

Now that we know the meaning I would explain further: Processing your request synchronously is a like reloading the page but with only the requested information having to be downloaded instead of the entire page. It does however still require that your visitor wait for the download to occur. While your visitors are used to having to wait for entire web pages to download, so unless the information you are requesting is particularly small so that it can be downloaded extremely quickly you run the risk of loosing visitors.

The preferred way to use Ajax therefore is to use asynchronous calls where ever possible so as to enhance your visitors experience with the web page and to avoid having the Ajax interfere with the operation of the page. 

Many beginners may fall into is to use synchronous Ajax calls where asynchronous calls are more appropriate (as they are most of the time). The reason for this is that synchronous calls are easier to understand how the processing works. The thing is that asynchronous calls actually work exactly the same way except for the processing not waiting for the response but instead just handling the response when it arrives.