If I didn't remember wrong... for FTP, there are two communication channels used, one is control channel, always activated by the client, means the client connects to the server. The other one is data channel, for data channel, you can either act as server (waiting for connection, thi sis the normal way, and you tell the server the port you are listening by sending PORT command), or you reqire the server to listen, and you connect to it (this is called passive mode).

Update: (copied from RFC 959)

control connection

         The communication path between the USER-PI and SERVER-PI for
         the exchange of commands and replies.  This connection follow
+s
         the Telnet Protocol.

      data connection

         A full duplex connection over which data is transferred, in a
         specified mode and type. The data transferred may be a part o
+f
         a file, an entire file or a number of files.  The path may be
         between a server-DTP and a user-DTP, or between two
         server-DTPs.

      data port

         The passive data transfer process "listens" on the data port
         for a connection from the active transfer process in order to
         open the data connection.
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