Hypertext Transfer Protocol Secure (HTTPS) is a communication protocol that enables secure communications over computer networks. In particular, HTTPS is widely deployed on the Internet. Actually, HTTPS is not even a protocol. HTTPS is the result of simply layering Hypertext Transfer Protocol (HTTP) on top of the SSL/TLS protocol. In other words, HTTPS is a result of the encapsulation of the HTTP protocol into SSL/TLS. Thus, HTTPS adds the security capabilities of SSL/TLS to the standard HTTP communications.
You can find more information about HTTPS on Wikipedia at http://en.wikipedia.org/wiki/HTTP_Secure.
Technical details on HTTPS are available in the appropriate RFC at http://tools.ietf.org/html/rfc2818.
Transport Layer Security (TLS) uses long-term public and private keys when exchanging a short-term session key to encrypt the data flow between the client and the server. Thus, the security of HTTPS is provided by SSL/TLS.