The internet relies on different protocols to transfer data in different forms.
You mentioned two of them - HTTP and HTTPS. HTTP is Hypertext Transfer Protocol, and it indicates that the data is intended to be opened and displayed in a web browser. The "S" in HTTPS indicates that the given data is being transferred securely.
FTP is File Transfer Protocol. This protocol indicates that the data being transferred is a file (perhaps an executable file or a large data file) that should be downloaded instead of opened and rendered in a browser.
SMTP stands for Simple Mail Transfer Protocol, and it is used to send outgoing mail. POP stands for Post Office Protocol, and it is used to retrieve mail from a mail server.
The "www" that is prepended to a web address is simply a deprecated way of indicating that a given web address points to a webpage that should be downloaded via HTTP. Now it operates as a subdomain, so technically www.hubpages.com is a sub-domain of hubpages, just like brianrock.hubpages.com. However, most webservers are set up to redirect addresses with the "www" to an address without it.