Interaction between the browser and the server is governed by the HyperText Transfer Protocol (HTTP), now an official Internet standard maintained by the World Wide Web Consortium (W3C). HTTP uses a simple request/response model: the client establishes a TCP connection to the server and sends a request, the server sends a response, and the connection is closed. Requests and responses take the form of messages. A message is a simple sequence of text lines.
HTTP messages have two parts. First come the headers, which hold descriptive information about the request or response. The various types of headers and their possible content are fully specified by the HTTP protocol. Headers are followed by a blank line, then by the message body. The body is the actual content of the message, such as an HTML page or a GIF image. The HTTP protocol does not define the content of the body; rather, specific headers are used to describe the content type and its encoding. This enables new content types to be incorporated into the Web without any fanfare.