What is HTTP?
The method by which World Wide Web pages are transferred over the network.
A Simple HTTP Example
The browser says:
GET / HTTP/1.0
And the server replies:
HTTP/1.0 200 OK
<title>Welcome to Boutell.Com, Inc.!</title>
The rest of Boutell.Com's home page appears here
The first line of the browser's request, GET / HTTP/1.0, indicates that the browser wants to see the home page of the site, and that the browser is using version 1.0 of the HTTP protocol. The second line, Host: www.boutell.com, indicates the web site that the browser is asking for. This is required because many web sites may share the same IP address on the Internet and be hosted by a single computer. The Host: line was added a few years after the original release of HTTP 1.0 in order to accommodate this.
The first line of the server's reply, HTTP/1.0 200 OK, indicates that the server is also speaking version 1.0 of the HTTP protocol, and that the request was successful. If the page the browser asked for did not exist, the response would read HTTP/1.0 404 Not Found. The second line of the server's reply, Content-Type: text/html, tells the browser that the object it is about to receive is a web page. This is how the browser knows what to do with the response from the server. If this line were Content-Type: image/png, the browser would know to expect a PNG image file rather than a web page, and would display it accordingly.
A modern web browser would say a bit more using the HTTP 1.1 protocol, and a modern web server would respond with a bit more information, but the differences are not dramatic and the above transaction is still perfectly valid; if a browser made a request exactly like the one above today, it would still be accepted by any web server, and the response above would still be accepted by any browser. This simplicity is typical of most of the protocols that grew up around the Internet.