Implicit Security: Implicit security automatically begins with an SSL connection as soon as the FTP client connects to an FTP server. In implicit security, the FTP server defines a specific port for the client to be used for secure connections.
Explicit Security: In order to establish the SSL link, explicit security requires that the FTP client issue a specific command to the FTP server after establishing a connection. The default FTP server port is used. This formal method is documented in RFC 2228.
Because Implicit SSL has a dedicated port strictly used for secure connections, Implicit SSL connections require less overhead when you establish the session.
You can think of Implicit SSL as "always on" and Explicit SSL as "turn on" when told to do so.