What is the difference between asynchronous and synchronous HTTP request?

Synchronous: A synchronous request blocks the client until operation completes. In such case, javascript engine of the browser is blocked.

Asynchronous An asynchronous request doesn’t block the client i.e. browser is responsive. At that time, user can perform another operations also. In such case, javascript engine of the browser is not blocked.

Check out Determining synchronous vs. asynchronous in web applications for previous discussion. In short:

Asynchronous APIs do not block. Every synchronous call waits and blocks for your results to > come back. This is just a sleeping thread and wasted computation.

Sachin Gandhwani's answer is very well explained in simple words. In case you are still not convinced with the difference of asynchronous HTTP request and synchronous HTTP request, you can read this - https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/Synchronous_and_Asynchronous_Requests