HTTP is an abbreviation for "HyperText Transport Protocol". HTTP is the Internet protocol for transferring hypertext files. It requires the host to use an HTTP server program, and the viewer to use a HTTP client program (see definition for "browser").
HTTPS is an encrypted form of the HTTP protocol generally used for sending sensitive data like creditcard numbers, bank transfers, and PayPal transactions between the webserver and the browser. HTTPS is also sometimes called "Secure HTTP" or "SSL" (an abbreviation which stands for "Secure Sockets Layer").