A good definition for the technical answer is here:
http://slacksite.com/other/ftp.html

What is boils down to is who attempts to make the data connection.
When files are transferred a data connection is made for the transfer, this a seperate connection from the control connection which initiates the actual transfer and sending of normal commands.

Sometimes firewalls can prevent the FTP server from initiating the connection so you need to tell the server that your client will initiate the connection.