Active (PORT) mode:
Passive (PASV) mode:
In passive (PASV) mode the FTP server specifies the data port that the FTP client will connect on and waits for the FTP client to connect. The FTP client will ask the FTP server for the server's IP address and port number by issuing the PASV command to the FTP server. This will usually solve the problem of firewalls filtering the incoming data connection.
For more detailed info, see our blog post: http://southrivertech.com/what-is-the-difference-between-active-and-passive-ftp/
Also, our QuickStart on configuring firewall security with Titan and Cornerstone: http://southrivertech.com/wp-content/uploads/qs_Cornerstone_Firewall.pdf