HTTPS is HTTP encapsulated in a SSL/TLS (Secure Socket Layer/Transport Layer Security) stream. This means that the stream is encrypted using a symmetric cipher for privacy. A MAC (Message Authentication Code), which is a one-way hash of the message, is used reliability and authentication.
in reply to HTTPS
See RFC 2246 if you're interested in the actual workings of TLS or RFC 2068 for details on HTTP/1.1 (note that these are the actual protocol specifications, so they're quite long and very technical)