HTTP, the HyperText Transfer Protocol, is the language or "protocol" that all web browsers speak when talking to web servers. And SSL, which stands for Secure Sockets Layer, is a protocol that provides secure communication. When two programs talk to each other using HTTP, but do it using SSL's secure communications instead of talking "in the clear," they are speaking HTTPS.
When two programs communicate via HTTPS, they need a way to verify each other's identity and agree on a method of encryption. They do this via SSL certificates. See what is an SSL certificate? for more information.
HTTPS URLs can be recognized by the additional s after http. By default, HTTPS communication happens on TCP/IP port number 443 instead of port 80.
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