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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.
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)