On NetTuts.com there's a new tutorial about what they think is the one thing every web developer should understand - the HTTP protocol and how its used in web-based communications.
HTTP stands for Hypertext Transfer Protocol. It’s a stateless, application-layer protocol for communicating between distributed systems, and is the foundation of the modern web. As a web developer, we all must have a strong understanding of this protocol. Let’s review this powerful protocol through the lens of a web developer. We’ll tackle the topic in two parts. In this first entry, we’ll cover the basics and outline the various request and response headers.
They cover some of the basics of the protocol first including its statelessness, the concept of URLs and the HTTP "verbs" (like GET, POST and DELETE). They also briefly cover the HTTP response codes (ex. 200, 304) and the flow of the request and response to and from the web server. They also look at some of the basic HTTP headers and the actual low-level text formats of the requests/responses.
There's a section at the end of the post that links you to a few tools that you can use to view the HTTP messaging happening in your requests, some of which you might already have. They also briefly cover the use of HTTP in a few libraries - ExpressJS, Ruby on Rails and jQuery's Ajax handling.