http://www.ussg.iu.edu/hypermail/GNU/Linux/net/9910.3/0146.html
> What's FTP Passive mode ?

Basically, FTP works as follows:

1) client connects to server, port 21 (ftp).
2) client sends commands like USER and PASS to login and other things.
3) When client wants to transfer data (for instance, doing an ls or
getting/putting a file) it does this in one of two ways:

Active:
The client sends the server an ip address and port to connect to.
The IP address is presumably its own, and the port is a random one that
the client has grabbed. So, using active mode, all data connections are
done in reverse... the server connects to the client. This causes
problems for firewalls, understandably.

Passive:
The client requests that the server specify an ip address and a
port to connect to, just as the client would in active mode. This means
that data connections are made in the same direction as the original
control connection, and so they present far fewer firewall problems.

As for where to find info on the web, you can look at the official
documentation:
ftp://nic.merit.edu/documents/rfc/rfc0959.txt
Or you can search yourself.