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