smtp STARTTLS vs SSL/TLS

http://stackoverflow.com/questions/5540374/starttls-vs-ssl-tls

The clearest explanation that I've read is from FastMail:

SSL and TLS both provide a way to encrypt a communication channel between two computers (e.g. your computer and our server). TLS is the successor to SSL and the terms SSL and TLS are used interchangeably unless you're referring to a specific version of the protocol.

STARTTLS is a way to take an existing insecure connection, and upgrade it to a secure connection using SSL/TLS. Note that despite having TLS in the name, STARTTLS doesn't mean you have to use TLS, you can use SSL.

Read the rest of the info they have, including a discussion of port numbers and POP/IMAP vs SMTP, here: https://www.fastmail.fm/help/technology_ssl_vs_tls_starttls.html