HTTP stands for Hypertext Transfer Protocol and is the main foundation for exchanging information throughout the World Wide Web. It determines the way the data is formatted and transmitted.
HTTPS is Hypertext Transfer Protocol Secure and is the result of the combination between HTTP and SSL. The function of HTTPS is the same as the one of HTTP, but the data is transmitted through a secure, encrypted connection. In order to set your site to open through HTTPS, you need to either configure it through shared SSL or have a private SSL.