I am fluent in HTML and PHP and slowly learning JavaScript however I have noticed that there is a huge hole in my knowledge when it comes to understanding how web software communications actually work.

I understand the flow of information across the net however I would like to learn about HTTP protocols to better understand how the data is actually sent back and forth through the internet to help me understand things like REST, HTTP Headers, AJAX requests, etc. However, I must be searching the wrong terms because I haven't been able to find a good description of HTTP protocols.

Any help is appreciated to point me in the right direction. Thanks!