HTTP stands for HyperText Transfer Protocol and is used to
transfer the data of a website from the server on which the website
is stored to the browser (i.e. Mozilla Firefox) of the user
requesting the webpage and back.
The "http://" prefix in a URL is used to tell the browser which
protocol (in this case HTTP) to use.
HTTPS is a secure version of HTTP (S meaning Secure) in which
all of your transmitted data is encrypted using SSL. It has the
prefix "https://"