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.
STARTTLS is a way to take an existing insecure connection and upgrade it to a secure connection using SSL/TLS.
Here is a valuable link that gives clear idea of SSL and TLS protocols.