File Transfer Protocol is a very old protocol, dating back to 1971, developed at MIT.
FTP is a very simple and efficient protocol for file transfers. It includes no encryption, and extremely minimal overhead overall. To date it is still probably the most popular protocol to transfer files in a client-server fashion. During the years FTP has gained only a few new features since it's inception.
Default port for FTP is 21.
While at times considered as a discreet protocol, it really is not. Servers configured for FXP only change their authentication behavior so that in active connection any IP may contact the server to the specified port for file transfer, therefore you can transfer data directly from server to server without the need for data to pass through the client computer. Especially useful for large amounts of data where transferring the data via client computer would be prohibitively slow.
FXP stands for File eXchange Protocol, despite it merely extends what FTP servers are capable of already, and actually is achieved by simply disabling a security feature (IP authentication). Unfortunately FXP can make the FTP server vulnerable for FTP Bounce attack, and therefore is disabled by default in most FTP servers.
FTP Client programs
Some popular FTP clients are: