Back in the 1990s, any time a website was slightly updated with new information, the entire webpage had to be refreshed and reloaded. Today, thanks to some techniques collectively known as Ajax, we can incrementally update parts of web pages without interrupting a user’s experience. You’ve probably seen this on Google when it gives you search suggestions as you type. Any time you see a portion of a web page update without the entire web page refreshing, that’s Ajax at work.
Synchronous programming is when you have to wait for previous parts of your program to finish running before moving onto another task. However, in web programming, the web application has to be listening for many possible events at once and then act accordingly.
XMLHttpRequest and jQuery
Many programmers use XHR indirectly through jQuery because the syntax is more convenient. Thanks to programmers before us, all you have to do to take advantage of Ajax in your projects is read through the jQuery and XHR object documentation.