HTTP, or hypertext transfer protocol, is the way a Web server communicates with browsers like Internet Explorer® and Mozilla Firefox®. HTTP lets visitors view a site and send information back to the Web server.
HTTPS, hypertext transfer protocol secure, is HTTP through a secured connection. Communications through an HTTPS server are encrypted by a secure certificate known as an SSL. The encryption prevents third-parties from eavesdropping on communications to and from the server.
NOTE: Only servers that have their own SSL can create HTTPS connections. A site's visitor cannot encrypt the connection.
If you have a hosting account with an SSL encrypting its connection, you can automatically redirect visitors to the HTTPS version of your site (more info).