Secure Socket Tunneling Protocol (SSTP) allows for lower layer protocols like PPP or L2TP to be tunneled over SSL. This is useful for remote access scenarios in which the user must connect back to the home network from unpredictable locations. SSTP is less useful for site-to-site VPNs, given that it contains additional overhead in the form of extra TCP and HTTPS headers.
SSTP is supported by Microsoft products in conjunction with the Routing and Remote Access (RRAS) service.
References:
http://blogs.technet.com/b/rrasblog/archive/2007/01/10/how-sstp-based-vpn-connection-works.aspx
http://www.windowsecurity.com/articles/Secure-Socket-Tunneling-Protocol.html