Sometimes when you try to connect to an FTP server using various types of FTP clients, some of these clients will work, while others won't.
In order to understand why, you need to know a little bit more about the internal workings of the FTP protocol.

The FTP handshake can work in two very different manners:
1. active mode
2. passive mode

A fairly technical explanation can be found here:
http://slacksite.com/other/ftp.html

Basically, the difference is that in active mode, both the server as well as the client will be required to open up ports to listen for incoming traffic.
This will cause problems if you are behind a firewall / NAT router. The most visible symptom is that your FTP client will cause the bult-in Windows XP SP2 firewall to pop up a dilog box informing you that the client is trying to open an inbound connection and asking you whether you want to unblock or keep blocking.

Some of the smarter FTP clients have a checkmark option or command line switch to enable passive mode for all FTP transfers.
In passive mode, only the server is required to open up ports for incoming traffic.
All browsers are by default configured to work in passive mode when used as FTP clients.
If you are behind a firewall / NAT router, passive mode gives you a better chance of connecting to an external FTP server.

I always use passive mode with any FTP settings that I configure. Active mode is slower (if it actually works).
Could you explain some advantages of using active mode with FTP? Seems unnecessary to do so, but I may be wrong.

Given that most of your users are behind firewalls, and the major platform vendors are switching to a more secure default setup with a hardened firewall, I would always configure an FTPd to support passive mode.

As I understand it, some servers require the client to be in active mode - rendering it not entirely a client side choice.
Never bothered to understand the difference, just switched to active when this was necessary. Thanks for clearing that up.

People who have troubles with ports on their NAT/router should visit http://www.portforward.com for an easy introduction on port forwarding and a guide for almost each router model.

Active mode is the legacy way of doing things. It is the way the FTP protocol was designed to work way back when the Internet was a small, friendly community of scientists, students, etc. Nobody called it active mode in those days because there was nothing to compare and contrast it with.
Passive mode was designed later as an alternative, when hackers became a real and constant threat.

The only advantage of active mode that I can think of is backwards compatibility.
For example, if a large corporation runs a big old mainframe with an FTP server that doesn't do passive mode and only needs to be available to FTP clients on the intranet.

I recently installed the FileZilla FTP server on my PC and, in order to get it to work, I not only had to forward the ports / port ranges for the control and data connections in order to get it to work in passive mode, but I also had to make my PC the default DMZ server in the router configuration, which now could make my PC somewhat more vulnerable, if I'm not careful.

So that would be another advantage of active mode - less vulnerability on the server side.

I need a script to set the "enable passive ftp" setting in internet explorer to enable. I am NOT a software person so while it might be easy to layout , to me it is GREEK :)

The default in IE7 is to NOT enable it and this is creating a problem for me to "explain" to people who are even worse off than I on how to simply go in and put a "check" in that box.

Thanks