FTP uses seperate connections, one for commands and another for data
In "Active" FTP when you connect to a server the server then makes a connection back on a different port. This works great when you have a public IP, but has problems crossing NAT since the FTP originating IP is likely to be private unless it was altered by NAT
In "Passive" FTP both channels originate from your PC, so there's little problem routing them to the server.
