From Seo Wiki - Search Engine Optimization and Programming Languages
Secure Hypertext Transfer Protocol (S-HTTP) is a little-used alternative to the HTTPS URI scheme for encrypting web communications carried over HTTP. S-HTTP is defined in RFC 2660.
Web browsers typically use HTTP to communicate with web servers, sending and receiving information without encrypting it. For sensitive transactions, such as Internet e-commerce or online access to financial accounts, the browser and server must encrypt this information.
HTTPS and S-HTTP were both defined in the mid-1990s to address this need. Netscape and Microsoft supported HTTPS rather than S-HTTP, leading to HTTPS becoming the de facto standard mechanism for securing web communications.