The protocol has three variants, differentiated by their version number. Version 0.9 is a very simple protocol which allows a simple octet-stream to be transferred without any meta information (e.g.: no headers are associated with requests or responses).

Version 1.0 introduces MIME-like headers in both requests and responses; headers are used to control both the protocol (e.g.: the "Connection" header) and to give information about the content being transferred (e.g.: the "Content-Type" header). This version has also introduced the concept of name-based virtual hosts.

Building on the success of HTTP/1.0, version 1.1 of the protocol adds persistent connections (also referred to as "connection keep-alive") and improved proxy control.